Full text of "Anatomy and histology of the mouth and teeth"

A GIFT FROM

Dr. F. M. Farnsworth

Buckhannon, W. Va,
1956

A?z

DO NOT CIRCULATE

ANATOMY AND HISTOLOGY

OF THE

MOUTH AND TEETH

BROOMELL AND FISCHELIS

ANATOMY AND HISTOLOGY

OF THE

MOUTH AND TEETH

I. NORMAN BROOMELL, D. D. S.

DEAN AND PROFESSOR OF PROSTHETIC DENTISTRY, DENTAL ANATOMY AND HISTOLOGY, DENTAL
DEPARTMENT, MEDICO-CHIRURGICAL COLLEGE OF PHILADELPHIA

AND

PHILIPP FISCHELIS, M. D.

ASSOCIATE PROFESSOR OF HISTOLOGY AND DEMONSTRATOR IN EMBRYOLOGY, MEDICO-CHIRURGICAL

COLLEGE OF PHILADELPHIA

THIRD EDITION, REVISED, WITH 357 ILLUSTRATIONS

PHILADELPHIA

BLAKISTON'S SON & CO

1012 WALN.UT STREET
1910

U3RARY
MENTAL SCHOOL
W.V.U.

OLD BOOKS

Printed by

The Maple Press

York, Pa.

TO
C. N. PEIRCE, D. D. S.

AS A SOUVENIR OF A LONG AND VALUED FRIENDSHIP AND A

TESTIMONY OF ESTEEM FOR HIS PROFESSIONAL

AND PRIVATE WORTH

THIS VOLUME IS RESPECTFULLY DEDICATED

BY THE AUTHORS

PREFACE TO THIRD EDITION.

In submitting to the dental profession a revised edition of this work,
the author has embraced the opportunity to make such corrections and
additions as were deemed advisable to further increase the value of the
book and to make it worthy of at least a portion of the approval accorded
the first edition. The general plan of the work has not been altered, and
while some exceptions have been taken to including within its pages
chapters devoted to the general anatomy of the mouth and its associated
parts, it was decided, after due consideration, to allow this to remain.

In the description of the teeth, the terms "superior" and "inferior"
have been changed to "upper" and "lower" and the term "palatal"
as applied to one of the tooth surfaces has been. discarded, and the word
"lingual" substituted.

Substantial additions have been made to Part II (Chapters I, II,
VII, etc.). It has been justly brought forward within recent years* that
there is a lack of interest and enthusiasm for collateral reading, special
studies, and original investigation not only among students, but also
among graduates of Dentistry. The experience gained in teaching has
suggested to the authors that this is due to the fact that dental students
use to a great extent text-books on Anatomy, Histology, and Embryology,
chiefly intended for medical students, and they generally believe that
most of the matter which is contained in these books is not intended for
them, having no practical application. Their attention is therefore
limited to those chapters which seem of importance to them, and thus an
intelligent understanding of the subject is not obtained. Recognizing
this fact, it seemed advisable to include in this Dental Text-book a de-
scription pertaining to general Cytology, general Embryology, and
Histogenesis, and a further presentaJ'crbr lie s»g nltlca 0IT i'ce of the facts thus
gained for an intelligent understanding of the f°*^imation of the organs of
the mouth.

It has been the aim to present the subject in a concise and attractive
form, including some illustrations which never before appeared in a
work designed for dental students. It is the hope of the authors that
the additions made will help to awaken an interest for collateral reading
and thus elevate the standard of education among dental graduates.
Philadelphia, November 15, 1909.

* Proceedings of the Meeting of the Institute of Dental Pedagogics.

vii

PREFACE TO THE FIRST EDITION

In the preparation of this work it has been the aim of the author to
systematically describe those parts of human anatomy which come
directly under the care of the stomatologist. In the earlier chapters,
which are devoted to a gross description of the mouth and those tissues
which enter into its construction, there has been no attempt at originality
other than in the arrangement, which includes a complete description of
one part before another is taken up.

In the writing and classification of the succeeding chapters the writer
has attempted what others, though wiser and better qualified, appeared
unwilling to undertake, and it is from the works of such as these that the
foundation for the present work has been derived.

Within the last few years the progress in nearly every branch of
dental education has made a work of this character an imperative want.
Dental therapeutics and dental chemistry have been well-nigh recon-
structed, while the investigations of the microscopist and physiologist have
brought forth many valuable revelations. Next in importance has been
the advance in, or rather the introduction of, technic teaching. Con-
siderable space has, therefore, been devoted to the surface anatomy of the
individual teeth, with a hope that it may be of value in dental anatomy
technic.

While in one or two instances the writer has departed from the field
assigned as a text, the parts thus included are so closely associated with
the mouth, both in a constructive and in a functional manner, that the
work would be lacking in completeness if they were omitted.

The illustrations are, with but few exceptions, the original work of
the author, being reproduced by photograph from the actual subject.
In many instances dissections were required to reveal the parts, this
being particularly true of those illustrations included in the chapter on
the Development of the Teeth, about one hundred dissections being re-
quired to accomplish the purpose. In preparing the illustrations de-
scriptive of the various surfaces of the individual teeth, the progress of
the work was materially interfered with by the difficulty experienced in
securing normal teeth out of the mouth; may their number ever grow less.

X PREFACE TO THE FIRST EDITION.

The author desires to thus publicly acknowledge obligations to the
works of Tomes, Black, Morris, Stohr, Klein, and Strieker. He is also
indebted to Prof. A. P. Brubaker and to Dr. C. P. Shoemaker for valuable
assistance rendered, and to P. Blakiston's Son & Co. for their many cour-
tesies during the preparation of the volume.

That there is a place for such a work as this purports to be the writer
has but little doubt; that the following pages will fill that demand is his
earnest desire, and it remains for the reader to ascertain how far these
demands have been met in the direction of its aim and endeavor.

TABLE OF CONTENTS.

PART I.— ANATOMY.

CHAPTER I.

General Description of the Mouth. — The Buccal Orifice; The
Lateral Walls of the Mouth; The Hard Palate, or Dome of
the Mouth; The Soft Palate and Fauces; The Floor of the
Mouth; The Tongue and its Attached Muscles, i

CHAPTER II.

The Bones of the Mouth. — The Superior Maxillae; The Palate

Bones; The Inferior Maxilla, or Mandible, 34

CHAPTER III.

The Temporomandibular Articulation. — The Muscles of Masti-
cation, 61

CHAPTER IV.

General Description of the Teeth; The Permanent Teeth; Classi-
fication, Surfaces, etc.; The Roots of the Teeth; The Dental
Arch, 72

CHAPTER V.
Occlusion of the Teeth, 85.

CHAPTER VI.
The Blood- and Nerve-supply to the Teeth, 91

CHAPTER VII.

Other Structures Within the Mouth.— The Gums; The Mucous
Membrane; The Alveolodental Membrane; Glands, Ducts,

etc., 99

Xll TABLE OF CONTENTS.

CHAPTER VIII.

A Description of the Upper Teeth in Detail. — Calcification, Erup-
tion, and Average Measurements; Their Surfaces, Ridges,
Fossae, Grooves, etc., no

CHAPTER IX.

A Description of the Lower Teeth in Detail. — Calcification, Erup-
tion, and Average Measurements; Their Surfaces, Ridges,
Fossae, Grooves, etc., 172

CHAPTER X.

The Pulp-cavities of the Teeth, 198

CHAPTER XL

The Deciduous Teeth, Their Arrangement, Occlusion, etc.; Cal-
cification, Eruption, Decalcification, Shedding Process, and
Average Measurements; Their Surfaces, Grooves, Fossae,
Ridges, and Pulp-cavities, 224

CHAPTER XII.

Development of the Teeth. — The Dental Germs, Enamel Organ,
and Dentin Organ; The Dental Follicle; Calcification, Erup-
tion, etc., 240

PART II.— HISTOLOGY AND HISTOGENESIS.

CHAPTER I.
General Cytology; General Embryology and Histogenesis, . . . 293

CHAPTER II.

Elementary Tissues. — -Epithelial Tissue; Connective Tissue;

Muscular Tissue; Nervous Tissue; Blood and Lymph, . . 308

CHAPTER III.

The Mucous Membrane of the Mouth. — Of the Lips; Of the
Cheeks; Of the Gums; Of the Roof of the Mouth, Hard and
Soft Palate; Of the Floor of the Mouth and the Tongue, . . 328

f> ' TABLE OF CONTENTS. xiii

CHAPTER IV.

Glands and Ducts of the Mouth. — Of the Lips; Of the Cheeks;
Of the Hard and Soft Palate; Of the Tongue; The Salivary
Glands, 339

CHAPTER V.

Muscular Tissues of the Mouth. — Of the Lips; Of the Cheeks;

Of the Soft Palate; Of the Tongue, 347

CHAPTER VI.

Tissues of the Teeth. — Enamel; Dentin; Cementum; The Tooth-
pulp; The Alveolodental Membrane, 350

CHAPTER VII.

Embryology of the Mouth and Teeth, 411

CHAPTER VIII.
Anomalies of the Teeth, 437

Index, 447

ANATOMY AND HISTOLOGY

OF THE

MOUTH AND TEETH

PART I. -ANATOMY.

CHAPTER I.

General Description of the Mouth. — The Buccal Orifice (the Lips). —
The Lateral Walls of the Mouth (the Cheeks).— The Hard
Palate or Dome of the Mouth.— The Soft Palate and Fauces.
—The Floor of the Mouth.— The Tongue and its Attached
Muscles.

The mouth (Fig. i) (stoma, pi. stomata) is the entrance or gateway
to the alimentary canal, and is situated between the superior and inferior
maxillary bones and their attached tissues. It contains the active organs
of mastication, the teeth, the organs of taste, of which the tongue is chief,
together with some of the parts which assist in articulate speech. Anato-
mists usually divide this cavity into two compartments, the teeth serving
to separate one from the other, the inner space being called the mouth,
while that between the teeth and lips or cheeks is known as the vestibule
of the mouth. In this description all that space bounded anteriorly by the
lips, posteriorly by the pillars of the fauces, and latterly by the cheeks,
will be considered as a single cavity, and the organs and structures con-
tained therein, together with all parts directly interested in its formation,
will constitute a text for this work. The entrance to the cavity of the
mouth is formed by a freely movable transverse orifice or slit, the buccal
orifice, while communication is made with the pharynx posteriorly
through the fauces. Entering into the construction of the mouth and
assisting in the performance of its functions are bones, ligaments, muscles,
blood-vessels, nerves, glands, ducts, etc., each of which will be described
in turn.

ANATOMY

THE BUCCAL ORIFICE,

or entrance to the cavity of the mouth, is a transverse opening somewhat
variable in extent, the extremities of which are known as the corners or
angles of the mouth. The orifice is bounded by two fleshy folds, the

Fig. i. — Outer Wall of Nasal Fossa, with Mouth, Pharynx and Larynx in Vertical

Section. (Dearer.)

a, Superior meatus; b, superior turbinate body; c, middle turbinate; d, inferior turbinate;
e, inferior meatus; g, tongue; h, posterior pillar of fauces; i, geniohyogtossus muscle; ;', geni-
ohyoid muscle; k, hvoid bone; I, mylohyoid muscle; m, thyrohyoid membrane; n, ventricle
of larynx; o, thyroid cartilage; />, diaphragma sellae; q, ravum sella;; r, sphenoidal sinus;
s, middle meatus; /, rhinopharynx; u, Eustachian orifice; v, hard palate;_w, soft palate;
x, uxula; y, anterior pillar of fauces; z, tonsillar fossa; aa, oropharynx; bb, epiglottis; cc, arye-
piglottic fold; dd, larvngopharynx; ee, suprarimal portion of larynx; ff, ventricular band.
gg, vocal band; /;/;, infrarimal portion of larynx; ii, I ricoid cartilage; ;'/- tracheal ring.

upper and lower lips (labia), the former usually being in the form of a
double curve, coming together at the median line and forming a small
teat or tubercle, while the latter is made up of a single curve extending
from angle to angle. While this general description applies to the

THE BUCCAL ORIFICE. 3

labial forms most frequently met with, this must not be taken for a con-
stant condition. In some instances the lips are thin, with straight paral-
lel margins, firmly set against the teeth, and seldom separated from
each other when at rest. In another class they are thick, full, and promi-
nent, with their margins strongly curved, resting lightly against the teeth,
and more or less separated from each other during rest. Accompanying
these extremes as well as the intervening conditions are various other
peculiarities, such as the color, the rigidity or flexibility of the muscular
structure, etc. The upper lip generally overhangs the lower, but in some
instances the lower lip is the most prominent. Externally the lips are
covered by the common integument, internally and over their contiguous
surfaces by a continuation of the integument, the mucous membrane.
Between the external and internal coverings and forming the substance
of these fleshy folds are muscular fibers in which are imbedded numerous
blood-vessels, nerves, and glands (labial glands). By the various muscles
which enter into their construction the lips are loosely attached to the
surfaces of the maxillary bones.

The integument, or external covering of the lips, is similar to the
skin covering other parts of the body. In the male it is subject to a
peculiar change and modification of its outer layer, resulting in the pro-
duction of a hairy growth.

The mucous membrane, or internal covering of the lips, the begin-
ning of which is strongly manifest by its bright-red color, is without moist-
ure on the contiguous surfaces, is extremely sensitive, and contains a
number of vascular papillae, many of which are accompanied by nerve
terminals. Mucous membranes are described as lining certain cavities or
tracts, as the digestive tract, the respiratory tract, and the genito-urinary
tract, and it is upon the contiguous surfaces of the lips that the digestive
tract begins. The line of junction between the integument and the mu-
cous membrane is quite variable in form, but usually corresponds with the
general curvature of the lips. Internally at the median line each lip is
provided with a pronounced fold of mucous membrane, which is attached
to the basal portion of the gum, the fir nu in of the lip (fraenum labium
superioris and inferioris), which, in a measure, check the movements
of the lips.

Muscles of the Lips.

The muscular fibers within the substance of the lips are principally
those of a single muscle, the orbicularis oris, but associated with it is a
portion of the elevator and depressor muscles of the lips, the levator

4 ANATOMY.

labii superioris alceque nasi, levator labii superioris, depressor labii inferi-
oris or quadratus menti, and the zygomaticus minor.

Orbicularis Oris. — This is the sphincter muscle which surrounds
and controls the buccal aperture. In form it is an oval sheet with the
long axis placed transversely, the fibers being continued from one lip to
the other by passing around the angles of the mouth. It is divided into
an internal or labial portion, and an external or facial portion. The
labial portion forms the red part of the lips, and has no bony attachment
except through the medium of the adjacent muscles. The external or
facial portion forms the deeper layer and blends with the surrounding
muscles, works in conjunction with them, and is provided with the follow-
ing small bony attachments: The nasolabial slips are attached to the
septum of the nose, other fibers are attached to the incisive fossa of the
superior maxilla over the position of the lateral incisor tooth, and to the
incisive fossa of the inferior maxilla near the socket of the lateral incisor
or cuspid tooth.

Structure. — The muscle consists of three sets of fibers, one of which
runs transversely, one in a vertical, and one in an anteroposterior direc-
tion. The transverse set is continuous with the fibers of the buccinator or
cheek muscle, and forms the greater part of the muscle. The red or
labial portion of the muscle is also formed from the same fibers, while the
vertical fibers form the superficial part of the facial portion and are
continuous with the fibers of the levator and depressor muscles. Some
of these latter fibers pass around the corners of the mouth, thus becoming
transverse, those from above passing to the lower lip, while those from
below pass to the upper lip. The anteroposterior fibers pass from
before backward between the transverse fibers, and unite the mucous
membrane to the skin. These are chiefly found in the labial portion of
the muscle.

Relations. — The inner margin of the superficial surface is closely
connected with the integument, while superimposed between this and the
outer portion is a layer of fatty tissue. Upon the deep surface lies the
mucous membrane of the mouth, separated from the muscular tissue by
blood-vessels, the mucous glands, and small salivary glands.

Action. — To bring the lips together, to draw the upper lip downward,
and the lower lip upward; to draw together the corners of the mouth;
to throw both lips outward; to draw them back against the teeth, and to
oppose the action of all other muscles that blend into it and inclining to
draw it in various directions.

Levator Labii Superioris Alaeque Nasi. — As its name implies, this

THE BUCCAL ORIFICE.

muscle is an elevator of the upper lip and the wing of the nose. It is one
of the superficial facial muscles, is thin and triangular, and is situated
by the side of the nose, extending from the infra-orbital ridge to the
upper lip.

Frontalis

Orbicularis oculi

Procerus
Quadr. labii sup.
pars angularis

Nasalis, pars transversa-
Dilator naris anterior—;
Dilator naris posteriori
Quadr. labii sup.
caput infraorbitale

Caput zygomaticum

Caninus -^hjl.
Orbicularis oris

Quadratua labii inferioris
Triangularis

Auricularis
superior

Auricularis
anterior

Fig. 2. — The Superficial Muscles of the Head and Neck. (Morris.)

Origin. — From the nasal process of the superior maxilla near its
orbital margin.

Insertion. — From its origin it passes almost directly downward,
dividing into two portions, the smaller of which is inserted into the nasal
wing, while the larger portion is prolonged downward, blending into the

6 ANATOMY.

orbicularis oris and levator labii superioris, and forming a part of the
substance of the upper lip.

Relations. — Superficially, by the integument; deeply, by the levator
anguli oris and compressor narium.

Action. — By its smaller and shorter portion to raise the wing of the
nose and to dilate the nostril; by its larger and longer portion to elevate
the inner half of the upper lip.

Levator Labii Superioris. — This muscle belongs to the superficial
layer, and derives its name from its action.

Origin. — From the facial surface of the superior maxilla, at a point
between the orbital cavity and the infra-orbital foramen. Also by the
attachment of a few fibers to the malar bone.

Insertion. — Passing downward and inward, it is inserted into the orbi-
cularis oris and the integument of the upper lip. Near its lower third
it joins the levator labii superioris al&que nasi, and acts in conjunction
with it. Occasionally it is reinforced by fibers from the orbicularis pal-
pebrarum, which it receives at its outer border.

Relations. — Superficially, by the orbicularis palpebrarum and the
integument; deeply, by the levator anguli oris; the compressor nasi at
its origin, and by the infra-orbital vessels and nerves.

Action. — To elevate the upper lip.

Levator Labii Inferioris. — This muscle, also known as levator
menti, lies immediately beneath the mucous membrane of the lower
lip, and can best be dissected by everting the lip and lifting off the
membrane.

Origin. — From the incisor fossa of the lower jaw at its upper border.

Insertion. — Into the integument of the skin.

Relations. — With the labial integument, the lower border of the orbi-
cularis oris muscle, and its superficial surface with the oral mucous mem-
brane. Deeply it is in close contact with the periosteum and the depres-
sor labii inferioris.

Action. — To raise and cause to protrude the integument of the chin.

Depressor Labii Superioris. — This small muscle with its fellow of
the opposite side is sometimes found within the mucous membrane form-
ing the frenum of the upper lip.

Origin. — It arises from the incisive fossa along its lower margin and
some of its fibers are attached to that part of the alveolar process closely
associated with the fossa.

Insertion. — From the point of origin the fibers pass upward and are
attached to the lower border of the nostrils and partition of the nose.

THE BUCCAL ORIFICE. J

Some of the fibers of this muscle are also attached to the integument cover-
ing the wing of the nose. The balance pass downward and mingle with
the fibers of the muscular structure of the upper lip.

Relations. — At its point of origin the fibers are closely associated with
the mucous membrane forming the gums. Above this the muscular
structure of the lip overlies these fibers. Deeply it rests upon the surface
of the superior maxilla, and joins its fellow of the opposite side at the
median line.

Action. — To depress the upper lip.

Depressor Labii Inferioris, or Quadratus Menti. — The name
of this muscle is derived from its form and action. It belongs to the
second layer of facial muscles, is quadrilateral in shape, and consists of
parallel fibers which meet above in the median line.

Origin. — At the outer aspect of the lower border of the inferior
maxilla, from a point near the symphysis to the space beneath the first
bicuspid tooth.

Insertion. — Its fibers pass upward and inward, and after uniting
with its fellow of the opposite side, blend into the body of the orbicularis
oris of the lower lip.

Relations. — By its superficial surface with the integument and a
portion of the depressor anguli oris; deeply, with the mental nerve and
vessels, a portion of the orbicularis oris, the mucous membrane lining the
lower lip, and the labial glands.

Action. — To draw down and somewhat evert the lower lip.

Zygomaticus Minor. — An extremely slender muscle belonging to
the superficial set of facial muscles. It is closely associated with a larger
muscle, the zygomaticus major, belonging to the angular series, to be
described in connection with the muscles of the cheek.

Origin. — From the anterior inferior part of one of the facial bones,
— the malar, — close to its junction with the superior maxilla.

Insertion. — It passes downward and forward, its fibers becoming
lost in the special elevator muscle of the upper lip about midway between
the median line and the angle of the mouth.

Relations. — Superficially, by the integument, by its deep surface with
the levator anguli oris, facial portion of the orbicularis oris, and the
infra-orbital branch of the facial nerve.

Action. — To elevate and somewhat evert the upper part of the lip.
The Blood-supply to the Lips.

The blood-supply to the lips is principally through the superior and
inferior coronary arteries, both of which are branches of the facial artery.

8 ANATOMY.

In addition to these the inferior labial artery and the sub-mental, also
branches of the facial, and the mental branch of the inferior dental artery
supply a part of the lower lip.

The superior coronary artery courses along the inferior margin
of the upper lip, between the mucous membrane and the fibers of the or-
bicularis oris muscle. At the median line it anastomoses with its fellow
of the opposite side.

The interior coronary artery, somewhat smaller than the superior,
supplies the lower lip by coursing through its substance in a manner simi-
lar to the superior coronary and also anastomoses with its fellow of the
opposite side at the median line.

Course of the Blood From the Heart to the Lips. — From the
heart to the aorta, to the common carotid, to the external carotid, to the
facial, to the superior and inferior coronary and the inferior labial arteries.
After passing through the labial capillaries the blood is returned to the
heart through the superior and inferior coronary veins, and the larger veins
of which they are branches.
Nerves of the Lips.

The general nerve-supply to the lips is principally by small branches
of the infra-orbital nerve for the upper lip, and by branches of the mental
nerve for the lower lip. The buccal and superior maxillary branches of
the lower division of the facial nerve supply the orbicularis oris muscle;
the upper division of the facial nerve sends branches which supply the
levator labii superioris alaeque nasi, as well as the levator labii superioris
and the zygomaticus minor, while the superior maxillary branch of the
lower division of the facial supplies the depressor labii inferioris.

THE LATERAL WALLS OF THE MOUTH.

The Cheeks {buccal).

The cheeks are continuous with and similar in structure to the lips,
being covered internally by mucous membrane and externally by the com-
mon integument. Immediately beneath the mucous membrane are a
number of transverse muscular fibers, covered externally by a layer
of subcutaneous fat, and lying between this and the integument other
muscular tissue, the fibers of which radiate in various directions, accord-
ing to the action of the muscle to which they bslong. Besides muscular
and fatty tissue, there are imbedded within the substance of the cheek
blood-vessels, nerves, and glands. The fatty tissue spoken of as interven-

THE LATERAL WALLS OF THE MOUTH. Q

ing between the muscular fibers gives to the cheek the fullness and rotun-
dity desired by so many but possessed by so few.

The integument, or external covering of the cheek, is similar in
structure to the skin covering other parts of the body, and, like the lips
in the male, is productive of a hairy growth.

The mucous membrane, or internal covering of the cheek, is similar
to that of the lips, containing numerous glands (buccal glands) which are
almost identical to, but smaller than, the labial glands. In addition to the
buccal glands which are distributed over the entire membrane, there are
about five of larger size, which open into the mouth in the region of the
molar teeth, and are called molar glands (see Glands of the Mouth).

The Muscles of the Cheeks (See Fig. 3).

The transverse muscular fibers referred to as being immediately
beneath the mucous membrane are those of the buccinator, a muscle
named from its action, that of being the chief muscle employed by the
trumpeter. External to the buccinator is the masseter, one of the muscles
of mastication, the elevator and depressor muscles of the angle of the
mouth, the levator anguli oris, and the depressor anguli oris, and the
dermal muscles, zygomaticus major and zygomaticus minor, and the
risorius.

Buccinator. — This muscle forms the greater portion of the lateral
wall of the mouth. It is deep-seated in the cheek, being one of the third
stratum of facial muscles.

Origin. — The fibers are distinct in their origin from a part of the
alveolar process of the superior maxillary bone, at a point immediately
over the second and third molar teeth, from the anterior border of the
pterygomaxillary ligament, a narrow band of tendinous fibers or raphe
extending from the pterygoid plate of the sphenoid bone to the mylohyoid
ridge of the inferior maxilla near the position of the third molar tooth.
Some of its fibers also arise from the outer wall of the alveolar process of
the inferior maxilla below the second and third molars.

Insertion. — The fibers pass forward and converge as they reach the
lateral margins of the orbicularis oris; here the fibers of the upper portion
pass downward and blend into the muscles of the lower lip, while the
lower fibers pass upward and blend into those of the upper lip. Those
fibers which arise from the inferior maxilla pass forward and also blend
into the lower lip.

Relations. — Superficially, by the skin and subcutaneous fat, the duct
of Stenson, the masseter muscle, a portion of the angular group, and the

ANATOMY.

facial artery and vein. Passing over it are branches of the facial and buc-
cal nerves, also a layer of deep fascia continuous with that which covers
the upper part of the pharynx. By its deep surface it is in relation with
the mucous membrane and buccal glands.

Temporal

Quadr.labii sup.
caput angularis

NasaliB, pars —*
transversa i

Caninus

Depressor

septi nasi

Nasalis, pars

alaris

Orbicularis oris

Zygomaticus

Posterior belly
of digastric

Triangularis

Quadratus la-

bii inferioris

Mentalis

Mylo-hyoid

Anterior belly
of digastric

Scalenus anterior

FlG. 3.— The Deeper Muscles of the Face and Neck. (Morris.)

Action.— To draw outward or backward the angles of the mouth,
thus enlarging the buccal orifice and pressing the lips tightly against the
teeth; to force the food between the occlusal surfaces of the molar and
bicuspid teeth during mastication; to diminish the concavity of the cheek,
compressing the air contained therein and forcing it forward. It be-

THE LATERAL WALLS OF THE MOUTH. II

comes an auxiliary in deglutition by shortening the cavity of the pharynx
from before backward, through its connection with the superior
constrictor.

Masseter.— This muscle is placed immediately external to the
buccinator, and is one of the principal muscles of mastication. It is
short, thick, and somewhat quadrate in form, and is composed of two
sets of fibers, superficial and deep. The fibers of the former are directed
obliquely downward and backward; those of the latter, which are much
shorter, pass almost vertically downward.

Origin. — The superficial layer, from the malar process of the supe-
rior maxilla, and from the anterior portion of the zygomatic arch of the
malar bone. The deep layer from the posterior third of the zygomatic
arch, as well as from the greater part of its inner surface.

Insertion. — The superficial fibers are inserted into the ramus and
angle of the inferior maxilla, and the deep fibers into the upper half
of the outer surface of the ramus.

Relations. By its external surface with the zygomaticus major,
risorius, and platysma myoides muscles, the parotid gland and its duct;
by the transverse facial artery, the facial vein, and facial nerve, and by
the integument. By its internal surface with the ramus of the inferior
maxilla, a mass of fat which separates it from the buccinator, and with
the temporal muscle. Its posterior margin is in relation with the parotid
gland, and its anterior with the facial artery and vein.

Action. — The principal action of this muscle is to close the jaw and
to draw it slightly forward. (For further description, see Muscles of
Mastication, part i, chap. in.

The Angular Series. — The remaining muscles of the cheek are
those of the angular series, or those muscles which are inserted into the
angle of the mouth, two coming obliquely from above, — the levator anguli
oris and the zygomaticus major, — one running almost horizontally for-
ward, — the risorius, — and one ascending from below, — the depressor
anguli oris.

Levator Anguli Oris. — This muscle, which receives its name from
its action, belongs to the second layer of facial muscles. It is formed in
the shape of a triangular sheet.

Origin. — From the canine fossa of the superior maxilla, immediately
below the infra-orbital foramen.

Insertion. — Passing downward and outward it is inserted into the
angle of the mouth, its fibers blending with those of the orbicularis oris
and the other angular muscles.

12 ANATOMY.

Relations. — Superficially, with the levator labii superioris, the zygo-
maticus minor, and the infra-orbital vessels and nerves; deeply, with the
facial portion of the orbicularis oris and buccinator muscles, and the
mucous membrane of the mouth.

Action. — Especially to elevate the angle of the mouth, and to assist
in drawing these angles inward, decreasing the size of the buccal orifice.

Zygomaticus Major. — This muscle, the companion of which has
been described in connection with the muscles of the lips, belongs to the
first facial layer. It is composed of a long, fleshy band of muscular fibers,
which run direct from their point of origin to their point of insertion.

Origin. — From the malar bone, in close proximity to the zygomatic
suture.

Insertion. — From its origin it passes obliquely downward to the
angle of the mouth, and blends into the fibers of the orbicularis oris and
depressor anguli oris.

Relations. — Superficially, with the skin and subcutaneous fat; deeply,
with the malar bone, the masseter, and buccinator muscles, the facial
and transverse facial arteries, the facial vein, and branches of the facial
nerve.

Action. — To draw upward and outward the angles of the mouth,
as in smiling or laughing. By contracting, it throws into prominence the
cheek tissues in front of the malar bone, and forces the lower eyelid up-
ward. When acting simultaneously with its fellow of the opposite side,
the buccal aperture is widened, and the upper lip is elevated, exposing
the superior teeth.

Risorius. — One of the superficial set of facial muscles, receiving its
name from its supposed action in laughter (ridere, to laugh). It is flat
and ribbon-shaped, and is frequently very small and poorly developed.

Origin. — From the deep fascia covering the masseter muscle and
parotid gland, some of its fibers occasionally arising from the mastoid
process of the temporal bone.

Insertion. — Passing transversely forward and inward to the angle of
the mouth, its fibers blend with those of the orbicularis oris, and the
depressor anguli oris.

Relations. — Superficially, with the integument and subcutaneous
fat; deeply, with the masseter and buccinator muscles, the facial artery
and vein, and branches of the facial nerve.

Action. — To draw the angles of the mouth directly outward, thereby
increasing the width of the buccal orifice.

Depressor Anguli Oris. — Also one of the superficial layer of facial

THE LATERAL WALLS OF THE MOUTH. 13

muscles, deriving its name in accordance with its action upon the angle
of the mouth. It is a triangular-shaped muscle with its base below, be-
coming narrow as it ascends.

Origin. — From the lower border of the inferior maxilla, and from its
external oblique line below the cuspid, bicuspid, and first molar teeth.

Insertion. — Passing upward and inward it is inserted into the integu-
ment at the angle of the mouth, its fibers blending into those of the muscles
previously described as coming together at this point.

Relations. — Externally, with the integument; deeply or internally,
with the depressor labii inferioris, the buccinator, and the inferior coro-
nary artery.

Action. — To draw down the angle of the mouth and to slightly ex-
tend it.

Blood-supply to the Cheeks.

The blood-supply to the cheeks is principally through the facial
artery and its direct branches, the superior and inferior coronary, the
transverse facial, and branches from the internal maxillary.

The Facial Artery and Branches. — The facial artery, also called
the external maxillary, enters the cheek after passing over the body of
the inferior maxilla at the anterior edge of the masseter muscle. It
courses obliquely forward and upward through the substance of the cheek,
until it reaches the inner angle or canthus of the eye, where it joins the
nasal branch of the ophthalmic artery, and is called the angular artery.
Near the center of the cheek the inferior labial artery is given off, which
passes forward and downward to the lower lip, but supplies a portion of
the cheek in so doing. Midway between the center of the cheek and the
angle of the mouth the superior and inferior coronary arteries are given off,
supplying that part of the cheek immediately adjacent to the angle of the
mouth, after which they pass on to supply the upper and lower lips. The
masseteric branch is given off in the immediate center of the cheek, at a
point immediately below the inferior labial, passes directly upward over
the masseter muscle, and anastomoses with branches of the internal
maxillary and transverse facial. There are also given off from the
main trunk near the center of the cheek the buccal branches, which pass
upward over the buccinator muscle, and also anastomose with branches
of the internal maxillary and transverse facial arteries.

The Transverse Facial Artery. — This is the largest branch of the
temporal artery. It is at first deeply-seated in the substance of the pa-
rotid gland, after leaving which it courses transversely over and supplies the

ANATOMY.

masseter muscle, sends off small branches which supply the integument
of the cheek, and anastomoses with the buccal, infra-orbital, and the

Orbicularis oculi muscl

Transverse facial artery

Zygomatieus minor
muscle

Zygomatieus major
muscle

Buccinator muscle
Masseteric branch

Masseter muscle

Stylo-pharyngeus
muscle
Stylo-glossus muscle

Ascending palat
branch

Tonsillar branch

Extent at maxillary
artery
External carotid
artery
Posterior belly of
digastric muscle

Lingual artery

Frontal branch of ophthalmic
artery

Nasal branch of aphtha,

artery

Angular artery

Levator labii super-
ioris alaeque nasi
muscle

Infraorbital artt ry

Levator labii super-
iors propriua

La 1 1 ■ ral nasal or/cry

Caninus muscle

Artery of septum
Superior labial
artery

Risorius muscle

Iuii i tor labia/ artery

Mi ntal branch of inferior

ah < olar ai U ry
Quadratus labii interioris
muscle
- Inferior labial artery
Triangularis muscle

Submental artery
Branches to submaxillary
aland

Anterior belly of digastric muscle

Mylo-hyoid muscle

us muscle

HYPOGLOSSAL XERVE

Fig. 4. — Scheme of the Right External Maxillary Artery. (Morris after Walsham.)

facial arteries. Besides the arteries already named, the deeper portions
of the cheek receive blood from two branches of the internal maxillary
artery, the masseteric branch and the buccal branch. The former supplies

THE INTERIOR OF THE MOUTH. 1 5

the masseter muscle and anastomoses with the masseteric branch of the
facial, while the latter supplies the buccinator muscle and anastomoses
with the buccal branches of the facial.

Course of the Blood from the Heart to the Cheeks. — From the
heart to the aorta, to the common carotid, to the external carotid, to
the facial and its direct branches, or from the external carotid to the tem-
poral, to the transverse facial and branches.

From the cheeks the blood is returned to the heart principally
through the facial vein, a division of the anterior superficial vein. It
enters the cheek at a point midway between the lower eyelid and the wing
of the nose, passes obliquely downward, being in close contact with the
anterior edge of the masseter muscle over the body of the lower jaw, join-
ing the internal jugular vein in the neck. The transverse facial vein
which follows the course of the transverse facial artery, and the supe-
rior and inferior coronary veins also collect and convey a portion of the
blood from the cheeks to the larger veins and thence to the heart.
Nerves of the Cheeks.

The nerve-supply to the cheeks is principally from the seventh or
facial nerve and its branches, the buccal branch supplying a greater part
of their substance. There are also a few fibers of the infra-orbital branch
of the seventh nerve distributed to the labiobuccal region. The buccal
branch of the lower division of the facial, also the buccal branch of the
inferior maxillary division of the fifth nerve, supplies the buccinator
muscle. The infra-orbital branch of the upper division of the facial nerve
supplies the zygomaticus major and the levator anguli oris; the buccal
branch supplies the risorius, and the supramaxillary branch of the lower
division of the facial nerve supplies the depressor anguli oris.

THE INTERIOR OF THE MOUTH.

For convenience of description the mouth may be divided into
two parts — a superior portion and an inferior portion. In dissection this
division may be accomplished by an incision beginning at the angles of
the mouth and carried backward and slightly upward through the sub-
stance of the cheeks until the temporomandibular articulation is reached.
After disarticulating this joint, another incision is made, beginning at the
joint on either side, carried downward and forward, then obliquely across
the throat, until the two come together at the median line. This latter
incision must be deep enough to completely sever the tissues of the throat.

The superior portion of the mouth contains the hard palate, or roof

1 6 ANATOMY.

of the mouth, the soft palate, and the sixteen upper teeth, firmly set in the
bone and surrounded by a dense fibrous tissue — the gums. The inferior
portion contains the tongue and its attached muscles, forming the. floor
of the mouth, the sixteen lower teeth and the gums surrounding them.

THE SUPERIOR PORTION OF THE MOUTH (Fig. 5).

The osseous framework, or base upon which this half of the mouth
is constructed, is composed of a part of four bones — the two superior
maxillary, or upper jaw bones, and the two palate bones (see Bones of
the Mouth, p. 34).

The Hard Palate, or Roof of the Mouth (Figs. 5 and 6).

This is formed by the union of the palatal processes of the superior
maxillary bones and the horizontal plates of the two palate bones at the
median line. It is limited in front and laterally by the margins of the al-
veolar process, or that portion of the bone which gives support to the
teeth, and ends posteriorly in an irregular border, to which is attached a
muscular, membrane-like curtain — the soft palate. The hard palate is
covered throughout by a thick and firm mucous membrane, seldom so
highly colored as that lining the lips and cheeks. The mucous mem-
brane is closely adherent to the bone through its common covering, the
periosteum. In the center of the hard palate is a ridge or fold of mucous
membrane, which follows the median line from before backward; this
is called the palatal or median raphe, and indicates the line of union
formed during the development of the parts. Anteriorly, the raphe ends
in a small papilla, which marks the opening of a canal in the bone — the
anterior palatal canal. Posteriorly, the raphe usually diminishes, in size,
but occasionally is well marked through the whole extent of the hard
palate. Near the center of the hard palate it frequently separates into
two or more smaller ridges, which are proportionately diminished in size,
and are continued backward side by side. On either side of this central
ridge, anteriorly, the mucous membrane presents a number of fantasti-
cally arranged folds, the palatal ruga (wrinkles). These folds are
usually quite numerous and prominent, but are occasionally but slightly
developed. The nature of these wrinkles is strongly indicative of the char-
acter or temperament of the individual; thus, in the four basal tempera-
ments they may be divided as follows: in the bilious, heavy and
strong, composed of angles rather than curves; in the nervous, few in
number, close together, not prominent, and composed of long curves;
in the sanguine, quite numerous, fairly prominent, well rounded and

THE INTERIOR OF THE MOUTH.

graceful in outline; in the lymphatic, few in number, flat, widely sepa-
rated, and but little curved. Acccompanying these varying conditions
in the rugae will be found a corresponding variation in the raphe. The
anterior and lateral margins of the mucous membrane covering the
hard palate form the palatal portion of the gums, gingiva (the gum).

Fig. 5. — The Superior Portion or Roof of the Mouth.

In figure 6 the hard palate is shown with its mucoperiosteum re-
moved. It will be observed that the bony plates are perforated by num-
erous small openings or foramina, through which the body of the bone
receives its nourishment, broken by depressions for the accommodation
of the various mucous glands, and traversed by longitudinal grooves
which give lodgment to blood-vessels and nerves.

The arch formed by the hard palate from side to side varies greatly

1 8 ANATOMY.

in form, imparting much knowledge in regard to the temperament of
the individual, and in a measure controlling the quality of the voice.
Thus, in the sanguine temperament the roof of the mouth presents
almost a perfect oval. In the bilious type it is comparatively high and
flat, extending from the base of one alveolar process to another, from
which point it descends abruptly to the necks of the teeth. In the ner-
vous type the roof is high and semi-elliptical or parabolic in shape, and
in the lymphatic it is low and segmental in form.

In the same illustration the union or suture between the four bones
may be seen at the median line. Near the anterior third of this central
suture is the opening of the incisive or anterior palatal canal, the anterior
palatal foramen, the location of which has been referred to in the descrip-
tion of the mucous membrane. Near the posterior border, and situated
within the suture which unites the superior maxillary bones with the palate
bones (the palatomaxillary suture), are two other foramina, the posterior
palatal; and immediately behind these, and separated by a thin ridge of
bone, are the accessory palatal foramina, these being in the tuberosity of
the palate bones. (For further description of these foramina, see Bones
of the Mouth, p. 34.) By the vessels and nerves which enter the hard
palate through these various foramina, the mucous membrane and
glands receive their blood- and nerve-supply.

Blood-supply to the Hard Palate. — This is principally derived
from the posterior or descending palatal branch of the internal maxillary
or deep facial artery, which passes downward in the posterior palatal
canal and emerges through the posterior palatal foramen. Immedi-
ately on reaching the palate it divides into an anterior and a posterior
branch, the former passing forward in a groove provided for it to the an-
terior palatal foramen, where it anastomoses with the nasopalatal artery.
The groove in which the artery lies in its passage forward is usually at
the base of the alveolar process, and in some instances is converted into
a canal for a part of its length. The posterior branches pass backward
and downward to supply the soft palate. In connection with supplying
the hard palate proper, this artery carries blood to the palatal alveolar
walls, to the mucous glands, the mucous membrane, and the gums.

Course of the Blood from the Heart to the Hard Palate.—
From the heart to the aorta, to the common carotid, to the external
carotid, to the internal maxillary, to the posterior or descending palatal
branch of the latter. From the hard palate the blood is returned to the
heart by the superior palatal and inferior or descending palatal veins, the
former following the course of the superior palatal artery, while the

THE INTERIOR OF THE MOUTH. 19

latter originates at a point near the junction of the hard and soft palates,
passes downward, and joins the facial vein below the body of the in-
ferior maxilla.

Nerves of the Hard Palate, — The nerves of the hard palate are
the anterior or large palatal and branches from the nasopalatal, both of
which are branches of the sphenopalatal (Meckel's) ganglion. The an-
terior palatal nerve arises from the inferior angle of the ganglion, passes

Fig. 6. — The Hard Palate, or Roof of the Mouth, with its Membranous

Covering Removed.

downward, accompanied by the descending palatal artery, through the
posterior palatal canal, from which it emerges at the posterior palatal
foramen. From this point it passes forward in a groove of the hard
palate, and joins the nasopalatal nerve as it emerges from the anterior
palatal foramen. Accompanying this nerve in its course through the
posterior palatal canal are other branches of the sphenopalatal ganglion,
which pass to the soft palate, and will be described in that connection.
The nerves of the hard palate are all sensory, and filaments are dis-
tributed to the mucous membrane and glands and to the palatal portion
of the gums.

THE SOFT PALATE (Figs. 6 and 7).

The soft palate is attached to the posterior border of the hard palate,
from which it is continued as a backward prolongation. Hanging down-

20 ANATOMY.

ward, with its free borders inclining backward, it may be considered as
forming a part of the posterior boundary of the mouth. It partially
separates the mouth from the nasal cavity and from the pharynx. It is
attached laterally to the walls of the pharynx, while its lower border is
free. The substance of the soft palate is composed of a number of thin,
but dense, muscular fibers, blood-vessels, nerves, and mucous glands, the
latter being similar to those of the hard palate. The anterior surface
of this muscular curtain is concave, directed forward and downward,
and is traversed by a median raphe. The posterior surface is convex,
directed backward and upward, and is continuous with the nasal cavity.
Suspended from the center of its free border is a small rounded or conic
membranous appendix, the uvula, and passing outward from the base
of this at each side are two curved muscular folds which extend outward
and downward, and are known as the pillars of the fauces. From the
position which these folds occupy they are divided into the anterior and
the posterior pillars of the fauces. The anterior pillar is formed from
muscular fibers which extend from the soft palate to the side and base of
the tongue (palatoglossus muscle), and is somewhat prominent as it passes
downward, outward, and forward. The posterior pillar approaches
more closely to its fellow of the opposite side than does the anterior.
It is formed of muscular fibers which extend from the soft palate above
to the pharynx below (palatopharyngeus muscle). It is somewhat con-
cave in its downward and backward course, and while closely united to
the anterior pillar above, is separated from it below, leaving a triangular
interval or niche in which is lodged a small, almond-shaped body, the
tonsil, the space being known as the tonsillar recess. The intervening
space — bounded by the margins of the soft palate above, the root of the
tongue below, and the pillars laterally — is called the isthmus of the fauces,
which establishes the communication between the mouth and the pharynx.
The free margins of the soft palate, assisted by the pillars of the fauces,
mark the posterior boundary of the mouth. The entire surface of the soft
palate and its prolongations, the pillars of the fauces, is covered with
mucous membrane, being continuous with that of the mouth on its an-
terior surface, and with that of the nasal cavity on its posterior surface.
Muscles of the Soft Palate.

On each side the muscles of the soft palate are the palatoglossus,
palatopharyngeus, levator palati, and tensor palati, together with the
azygos uvulce.

Palatoglossus. — A small fasciculus of fibers, somewhat cylindric
in form, expanding at either end into a thin sheet. It is named from its

THE INTERIOR OF THE MOUTH.

attachment to the soft palate and to the tongue. It is the prominence of
this muscle, together with its covering of mucous membrane, that forms
the anterior pillar of the fauces.

M. uvulae
Hamular process

Fharyngo-
palatiuus

AUDITORY OR

EUSTACHIAN TUBE

Levator veli
palatini

Pharyngo-palatinus

Constrictor pharyngis
superior

Crico-arytaenoideus
posterior

Thyreoid cartilage

Cricoid cartilage

TRACHEA

ESOPHAGUS

Fig. 7.— View of Muscles of Soft Palate, as seen from within the Pharynx.
{Morris Modified from Bourgery.)

Origin. — By a thin muscular sheet from the under surface of the
aponeurosis of the soft palate near the median line, its fibers uniting

2 2 ANATOMY.

with those of the opposite side. It passes downward in front of the ton-
sil and against the pharyngeal wall.

Insertion. — Into the side and base of the tongue.

Relations. — Superficially, it is covered by the mucous membrane
of the soft palate and tongue; deeply, in contact with the aponeurosis
of the soft palate, the superior constrictor muscle of the pharynx, and
one of the muscles of the tongue — the hyoglossus.

Action. — The lateral walls of the soft palate are drawn down, and
the sides of the tongue are drawn upward and slightly backward. Acting
in conjunction with the palatopharyngeus, the opening of the fauces is
constricted.

Palatopharyngeus. — This muscle — also named from its attach-
ments — is broad above, where it forms the greater part of the lower
half of the soft palate. Near the median line a few of its fibers blend
with those of its fellow of the opposite side.

Origin. — By two heads from a point near the raphe or median
line of the soft palate, passing downward and slightly backward,
forming, with its covering of mucous membrane, the posterior pillar of
the fauces.

Insertion. — Into the posterior border of the thyroid cartilage, and to
the inner surface of the lower part of the pharynx.

Relations. — In the soft palate, superficially, with the mucous mem-
brane, both anteriorly and posteriorly; above, with the levator palati;
and beneath, by the mucous glands. In the posterior pillar it is sur-
rounded with mucous membrane, and in the pharynx by the constrictor
muscles of the pharynx and the mucous membrane.

Action. — To constrict the opening of the fauces, by bringing together
the posterior pillars, thus depressing the soft palate and elevating the
pharynx. It controls the position of the soft palate during respiration,
and elevates the pharynx during deglutition.

Levator Palati. — This is a moderately thick muscle, and derives
its name from its action upon the soft palate.

Origin. — By a short tendon from the under surface of the petrous
portion of the temporal bone, and from the posterior ahd inferior aspect
of the cartilage of the Eustachian tube.

Insertion. — After passing downward by the side of the posterior nares
it is inserted into the median line of the soft palate, where its fibers
unite with those of its fellow of the opposite side.

Relations. — Externally, with the tensor palati and superior con-
strictor muscles; internally and posteriorly, with the mucous membrane.

THE INTERIOR OF THE MOUTH. 23

Action. — To raise the soft palate, bringing it against the posterior
wall of the pharynx.

Tensor Palati. — This is a slender and flattened muscular sheet,
and receives its name from its action upon the soft palate.

Origin. — From the scaphoid fossa at the base of the internal ptery-
goid plate and the spinous process of the sphenoid bone; also from the
outer side of the anterior aspect of the Eustachian tube.

Insertion. — After descending between the internal pterygoid muscle
and the internal pterygoid plate, and winding around the hamular process
of the latter, it is inserted into the transverse ridge on the horizontal
portion of the palate bone, and at the median line of the soft palate,
where it is continuous with the aponeurosis of the opposite side.

Action. — To tighten or spread the soft palate laterally, forming a
septum between the posterior nares and the pharynx. It also opens the
Eustachian tube during deglutition.

The azygos uvulae (so named because it was at one time supposed
to be a single muscle) is composed of a pair of small muscles which orig-
inate from the aponeurosis of the soft palate and the nasal spine of the
palate bone. They pass downward and form or are inserted into the
uvula.

Relations. — Anteriorly, with the levator palati, palatoglossi, and a
part of the palatopharyngei; posteriorly, to the mucous membrane.

Action. — To shorten or draw up the uvula.
Blood-supply to the Soft Palate.

The posterior or descending palatal branch of the deep facial artery.
after emerging from the posterior palatal canal, sends its posterior divi-
sion backward and downward to the soft palate, in the substance of which
they anastomose with the ascending palatal artery. After passing over
the superior border of the pharynx, the ascending pharyngeal artery sends
off branches which are distributed to the soft palate. A few branches of
the superior palatal branch of the internal maxillary and a few twigs
from the lingual artery also convey blood to the parts.

Course of the Blood from Heart to the Soft Palate. — From the
heart to the aorta, to the common carotid, to the external carotid, to the
facial or lingual, to the various branches named above, to the soft palate.
The return of the blood to the heart is principally through the superior
and inferior or descending palatal veins, both of which closely follow the
course of the arteries of the same name.

Nerves of the Soft Palate. — The small, or posterior, palatal, the
external palatal (both of which are branches of Meckel's ganglion).

ANATOMY.

branches of the glossopharyngeal nerve, and the following nerves which
supply the various muscles: filaments from the pharyngeal plexus to the
palatoglossus and palatopharyngeus, branches of the Vidian to the leva-
tor palati and azygos uvula?, and from the mandibular division of the fifth
nerve to the tensor palati.*

THE INFERIOR PORTION OR FLOOR OF
THE MOUTH (Fig. 8).
This half of the cavity of the mouth contains the tongue and its
attached muscles, the sixteen lower teeth firmly implanted in the jaw, and

Fig. 8. — The Inferior Portion or Floor of the Mouth.

the gums covering the alveolar walls. The base or osseous framework
upon which this portion of the mouth is constructed is principally made
up of a single bone, the inferior maxillary, mandible, or lower jaw bone

* A description of the upper teeth will be found in another chapter

THE INTERIOR OF THE MOUTH. 25

(see Bones of the Mouth, p. 34). The hyoid bone, situated between the
angles of the mandible in the upper part of the neck, and at the base of
the tongue, giving attachment to many of the muscles about the floor of
the mouth, may also be considered in this connection. The floor of the
mouth is bounded anteriorly by the lower lip, laterally by the cheeks,
and below by the muscles attached to the external and internal oblique
lines of the mandible.

THE TONGUE (Fig. 9).
The tongue is a freely movable, highly sensitive, muscular organ. It

True Vocal Cord Posterior Wall of the Pharynx Corniculum Laryngis

False Vocal
Cord.

Tonsil

Adenoid Tissue
at Base of Tongue

Foramen Caecum

Cuneiform
Cartilage

Epiglottis

Median Glosso-
epiglottidean Fold

Fungiform Papillae Circumvallate Papillae
Fig. 9. — Superior Aperture of Larynx. (Deaver.)

assists in the function of speech, participating, also, in the special sense

ANATOMY.

of taste, and in mastication and deglutition. The organ is attached
posteriorly to a U-shaped bone, the hyoid bone, which of itself is movable,
and is placed in the neck between the angles of the lower jaw and the
thyroid cartilage.

The tongue is suspended and kept in its position in the mouth by
numerous muscles, some of which are attached to the base of the skull,
and others to the lower jaw and hyoid bone.

The size of the tongue bears little or no relation to the size of the indi-
vidual, but is proportioned to the capacity of the alveolar arch, which space

it completely fills when at rest. The
shape of the tongue is controlled by
the shape of the alveolar arch; thus,
when the arch is contracted, narrow,
and pointed, the margins of the
tongue, when at rest, will assume
that form (a, Fig. 10); but when the
arch is broad and rounded anteriorly,
the margins of the tongue will also
be broad and rounded (b, Fig. 10).

The substance of the tongue is
chiefly composed of muscular fibers,
which are arranged in a complicated
manner, crossing one another at various angles, thus making the
movements of the organ exceedingly varied and extensive. Fibrous,
areolar, and fatty and glandular tissue enter into its structure, and it is
freely supplied with blood-vessels and nerves. Its free surface is covered
by a specialized mucous membrane, and over its entire surface art-
numerous mucous follicles and glands.

Before continuing the description of the tongue, it will be necessary
to name its parts. The upper surface, or that facing the roof of the
mouth, is the dorsum; those portions directed toward the cheeks are
known as the sides of the tongue; while the hem which unites these two
borders at the median line, and extending a short distance backward
on either side, is the tip. That portion between the frenum and extend-
ing back to the pillars of the fauces is the base, while that part of the dor-
sum immediately posterior to the tip is the post-lip, that region which
lies between the post-tip and the base being the prebase. The dorsum,
sides, and tip are free, while the base is attached by muscles to the lower
jaw and hyoid bone.

From the base to the epiglottis is a fold which serves to limit the

Fig. io.

THE INTERIOR OF THE MOUTH. 27

movement of the latter organ, and to the sides of the base the pillars of
the fauces are attached. Under the anterior free extremity in the median
line is a fibrous muscular lamina or ligament, the frenum (frenum linguae),
which connects this part of the organ with the lower jaw and marks the
anterior border of the base of the tongue. The tongue is divided through
its anterior two-thirds by a slight longitudinal furrow, the median raphe,
which ends posteriorly near a small foramen, not constant in the adult
tongue, but plainly observed in the fetus or infant, the foramen ccecuni.
This foramen represents the upper termination of the thyreoglossus duct.

Papillse of the Tongue. — Over the anterior two-thirds of the dor-
sum and the sides and tip of the tongue are a number of small, soft,
conic eminences, wh ch are known as the papillae of the tongue. These
are most numerous over the anterior part of the dorsum, and at the back
they are covered and partly hidden by an epithelial coating. In general,
the papilla? are quite similar to those of the integument, not being com-
pound organs in their vascular and nervous supply. In consequence of
their variation in form, and arrangement the papillae are variously
named. The largest papillae, being arranged like the letter V, are called
the circumvallate or jalciform; those of medium size, the fungiform, are
so named from their resemblance to a young mushroom; and the smallest
and most numerous are known as the conic or filiform papillae. Each
papilla presents a broad, free end, and is attached by a constricted base,
which rests in a small, cup-like concavity, about the margins of which is a
well-formed circular rim. Beneath the thick epithelium of these parts
are numerous secondary papillae, and about the base of each papilla
are the openings of one or more glands.

The circumvallate or jalciform papillae (Fig. 9) form a V-shaped line
at the posterior boundary of the dorsum. They are few in number
(varying from six to twelve), but are largest in size, not infrequently meas-
uring 1/4 of an inch in diameter. These papillae are generally regarded
as being gustatory, or directly interested in the sense of taste. Each
papilla is capped with a small secondary papilla.

The fungiform papillae (Fig. 9), of medium size, varying from 1/20
to 1/50 of an inch in diameter, are scattered over the dorsum, sides, and
tip of the tongue at irregular intervals, and are much more highly colored
than the smaller papilla? which surround them. They vary greatly in
number, and being principally gustatory, account in a great measure for
the diversity in the acuteness of the sense of taste in different individuals.
These papilla 1 , like the circumvallate, are capped with smaller secondary
papilki?.

28 ANATOMY.

The conic ox filiform papillae (Fig. 9) are the smallest and most numer-
ous, and are thickly scattered over the entire surface of the dorsum in
front of the circumvallate as well as over the sides and tip of the tongue.
They are placed closely together, and with such regularity that they
fairly ridge the tongue with delicate lines, which run parallel with the
circumvallate in that region, but as the tip is approached they become
transversely inclined. These papillae are generally regarded as being
tactile or directly interested in the sense of touch, and are concerned in
directing the movements of the food during mastication. They also pos-
sess secondary papillae upon their surfaces.

Immediately posterior to the circumvallate papillae are two shallow
grooves which follow the V-shaped line of the papillae and unite at the
foramen caecum. These grooves serve to indicate the line of junction
between the anterior and posterior portions of the tongue. The latter not
being within the cavity of the mouth will not be described.

Muscles of the Tongue (Fig. 11).

The muscles of the tongue are both extrinsic-outward or external, and
intrinsic-inherent, inward, or special.

The extrinsic muscles include those which have their origin from
the base of the skull, the hyoid bone, or the mandible, and are the hyo-
glossus, geniohyoglossns, styloglossus, palatoglossus, and a few fibers of
the superior constrictor of the pharynx. The intrinsic muscles which
make up the bulk of the tongue are two in number, the superior lingualis
and inferior lingualis.

Hyoglossus. — As its name implies, this muscle extends from the
hyoid bone to the tongue. Its fibers are so arranged that they form a
thin square sheet.

Origin. — It arises from the whole length of the upper border of the
great cornu, from the body, and by a few fibers from the lesser cornu of
the hyoid bone. At their point of origin the fibers are in the form of a
thin sheet, and ascend toward the tongue almost parallel to one another,
but before reaching the tongue the anterior fibers pass slightly forward,
and at the upper margin of the side of the tongue bend inward and join
the fibers of the superior lingualis. In their distribution to this part of
the tongue they form a kind of submucous covering to the organ.

Insertion. — Into the posterior half of the side of the tongue, between
the styloglossus and superior lingualis muscles.

Relations. — Externally, with the digastricus, styloglossus, stylohyoid,
and mylohyoid muscles, the lingual and hypoglossal nerves, Wharton's

THE INTERIOR OF THE MOUTH.

2 9

duct, and the sublingual gland. Internally, with the lingualis, geniohyo-
glossus, and middle constrictor of the pharynx muscles, the lingual artery,
and the glossopharyngeal nerve.

Action. — To extend the tongue and to draw it backward, also to

Stylo -glossus

DORSUM OF TONGUE

Genio-hyo-glossus
Genio-hyoid

Cut edge of mylo-hyoid

EPIGLOTTIS
(indicated by dotted tines)

GREATER CORNU OF HYOID BONE

STYLOID PROCESS

Stylo-hyoid

POSTERIOR PORTION
OF TONGUE

Tjjr - Stylo-pharyngeus

Hyo-glossus

Thyro-hyoid
ligament

CARTILAGO TRITICEA

Thyro-hyoid
membrane

THYROID CARTILAGE

Median portion of
crico-thyroid
membrane

CRICOID CARTILAGE
FIRST RING OF TRACHEA

rn ww r ff n

Fig. 11. — Side View of the Tongue, with its Muscles. (Morris.)

draw downward the sides of the tongue, making its dorsum more convex
transversely.

Geniohyoglossus (Fig. 11). — This muscle also receives its name
from its three points of attachment, the chin internally, the hyoid bone,
and the tongue. It is a triangular-shaped muscle, narrow and pointed
at its attachment to the mandible, and broad and fan-shaped on approach-
ing the tongue. Being near the median line, it is separated from its

3°

ANATOMY.

fellow of the opposite side by a thin layer of connective tissue, the septum
of the tongue.

Origin. — It arises by a short tendon from the upper genial tubercle
of the lower jaw, from which point its fleshy fibers diverge fan-like to its
extensive insertion.

Insertion. — To the whole length of the tongue from base to apex

Vertical and Trans-
Mucous Submucous Superior Lingualis ve . rse Musc " lar Fibers

Membrane Tissue Muscle

Inferior Lingualis Muscle

Ranine Arteries Vena Comes
Intrinsic Muscular Fibers

Fig. 12.— Transverse Section of One-half of Tongue. {Denver.)

immediately external to the median line, into the body of the hyoid bone,
and by a few fibers into the side of the pharynx.

Relations. — By its inner surface, with the septum of the tongue and
its fellow of the opposite side; by its outer surface, with the hyoglossus,
mylohyoides, styloglossus, and lingualis muscles, sublingual gland, lin-
gual artery, and hypoglossal nerve. Superiorly, with the mucous mem-
brane of the floor of the mouth; inferiorly, with the geniohyoid muscle.

THE INTERIOR OF THE MOUTH.

3 1

Action. — Its anterior fibers assist in drawing back the tip of the
tongue, its posterior fibers throwing forward and protruding the tongue.
This muscle also depresses the center of the dorsum longitudinally,
making it concave transversely, and some of the lower fibers which are
attached to the hyoid bone elevate the bone and assist in raising the tongue.

Styloglossus. — Also named from its attachment, is a long fan-shaped
muscle, somewhat compressed laterally.

FR/ENUM LING'Jyfc

Linguahs inferior

Hyo-gloseus

Genio-hyoid rj/

Mylo-hyoid, reflected

Stemo-hyoid

- LingualiB inferior

Genk-hyo-glossus

Stylo-glosBus

Hyo-glossus
BODY OF HY010 BONE
Genio-hyoid

THYROID CARTILAGE

Fig. 13. — Under Surface of the Tongue with Muscles. {Morris.)

Origin. — From its point of origin at the tip of the styloid process of
the temporal bone, and from a portion of the stylomaxillary ligament, it
passes with a long curve, forward, slightly downward, then upward and
inward to its place of insertion at the side of the tongue.

Insertion. — Upon reaching the side of the tongue it divides into two
portions, the fibers of one portion passing transversely inward, while the
others pass longitudinally along the side of the tongue.

Relations. — Externally, with the internal pterygoid muscle, parotid

32 ANATOMY.

and sublingual glands, lineal nerve, and the mucous membrane of the
mouth; internally, with the superior constrictor and hyoglossus muscles,
and with the tonsil.

Action. — To draw the tongue backward and to produce a transverse
concavity to its upper surface by elevating its sides.

Superior Lingualis. — This is one of the intrinsic muscles, and is
situated immediately beneath the mucous membrane, extending from the
base to the tip of the organ.

Inferior Lingualis (Fig. 13). — This muscle is placed near the under
surface of the tongue, and is composed of two bands which extend from
base to apex, some of its fibers being attached posteriorly to the hyoid
bone, and in passing forward are placed between the hyoglossus and genio-
hyoglossus. Anteriorly, its fibers blend with those of the styloglossus.

Many of the fibers of this muscle run transversely and are placed
between the two former intrinsic muscles. These, together with some
fatty tissue compose the greater part of the substance of the tongue.
The fibers are attached at the median line to the fibrocartilaginous sep-
tum of the tongue, and laterally to the mucous membrane. In connection
with the transverse fibers there are a few placed vertically, which pass by
long curves from the dorsum to the under surface of the tongue.

Blood-vessels of the Tongue (Fig. 14).

This organ receives its blood principally through the lingual, facial.
and ascending pharyngeal arteries. The lingual artery arises from the
front of the external carotid near the facial, and often as a common trunk
with it. From its point of origin to the tongue it is divided into three
portions, the first or oblique, the second or horizontal, and the third or
ascending, and it is this latter portion which directly supplies the tongue.
Ascending tortuously beneath the hyoglossus muscle, it reaches the under
surface of the tongue, and, lying between the lingualis and hyoglossus
muscles, it is continued to the under surface of the tip of the tongue, at
which point it is called the ranine artery. At a point about corresponding
with the posterior margin of the hyoglossus muscle a branch is given off
(the dorsalis lingua), which passes almost directly upward, and, after
dividing into two or more small branches, supplies the back part of the
dorsum of the tongue and the mucous membrane about the circumvallate
papillae. At the anterior border of the hyoglossus muscle another branch
is given off (the sublingual artery) supplying the anterior muscular struc-
ture of the floor of the mouth. The facial artery by one of its muscular
branches supplies the styloglossus muscle.

THE INTERIOR OF THE MOUTH.

Course of the Blood From the Heart to the Tongue. — From the
heart to the aorta, to the common carotid, to the external carotid, to the
lingual artery, and its smaller branches to the tongue. From the tongue
the blood is returned to the heart principally through the lingual vein,
which begins at the ranine vein beneath the tip of the tongue, passes back-
ward under cover of the mucous membrane, following the course of the
lingual artery until the hyoglossus muscle is reached, beyond which point

Posterior br. of
descending palatine A

Palatine br. of
a,s cen dm <j pharyngeal A- -Al/

Ascending pharyngea

A seen ding pa la I in e br
of facial A

Tonsillar br.

of facial A.

Stylo pharyngeus M-

Facial A.

Middle conslriclorM-
Dorsalis linauae A.

Lingual A-
£x tern a I caro lid A .

-Descending palatine .4.

-Anterior br. of descending palatine A.
z Stylo-glussusM.

,1'alato -glossusM.

, Tonsillar br. dorsalis linguae A .

Superior thyroid

Infra - hyo id br. ofsfyp. thyroid A ^ffL

astricM.

oftingualA. ^lyiv-nyoid M

Fig. 14. — Arteries of Tongue and Tonsil

ry of fraenuni
Submental A.
Gewo-hyoid M.
Genio -hyo-glossus M
Sublingual A .

(Deaver.)

the fibers of the muscle separate the artery from the vein. After receiving
the sublingual and dorsalis linguae veins, the course of which corresponds
to the arteries of the same name, the vein passes backward and down-
ward and empties into the internal jugular.

Nerves of the Tongue.

The mandibular division of the fifth nerve by its lingual branch sup-
plies the papillae of the anterior portion and sides of the tongue, while the
lingual branch of the glossopharyngeal supplies the circumvallate papillae,
the base, and posterior sides. A few branches of the superior laryngeal
are distributed to the back part of the root of the organ. The motor
nerve of the tongue is the hypoglossal or ninth, supplying both the extrinsic
and intrinsic muscles.*

* A description of the lower teeth will be found in another chapter.

CHAPTER II.

The Bones of the Mouth: The Superior Maxillae, The Palate Bones,
The Inferior Maxilla or Mandible.

SUPERIOR MAXILLARY BONES.

The superior maxillary bones, two in number, one on each side of
the median line or center of the face, are irregular in shape, and may be
classed as the largest bones of the face, with the probable exception of
the mandible or inferior maxilla. From the central position which they
occupy they contribute largely to the bony framework of this portion of
the skull. They are not only instrumental in forming the major portion
of the roof of the mouth or hard palate, but assist in the formation of
the floor of the orbit, and the sides and base of the nasal chamber. They
furnish a solid and firm foundation for the sixteen upper teeth, and by
their variety in form contribute much to the character and quality of the
voice. The outer or facial surface of these bones provides attachment
for numerous muscles. Each superior maxillary bone presents for
examination a body, four surfaces, and four processes. The body may be
described as forming an irregular triangle, its general contour depending
much upon the temperament of the subject and consequent character
of the teeth. Within the body of the bone is an irregular cavity, the maxil-
lary sinus or antrum of Highmore.

The four surfaces of the body of the bone are the superior or orbital,
the lateral or facial, the proximal or nasal, and the posterior or zygomatic.

The Superior or Orbital Surface, which assists in forming the
greater portion of the floor of the orbit, is slightly concave over its anterior
two-thirds, and somewhat convex over the remaining or posterior third.
The three borders of this surface form almost an equilateral triangle, and
are named, as indicated by their location, the anterior, the posterior, and
the mesial or proximal. The anterior border is convex from before back-
ward and slightly concave throughout its length. That portion which
forms a part of the lower border of the completed orbit is smooth, while
the remaining portion is roughened to form an articulation with the malar
bone. The posterior border extends from the center of the malar process
backward and inward to the orbital process of the palate bone, which

SUPERIOR MAXILLARY BONES. 35

articulates with the superior maxillary at this point. A portion of this
border, together with the orbital process of the palate bone, is instrumen-
tal in forming the anterior boundary of the sphenomaxillary fissure.

The mesial or proximal border is marked by an irregular thin edge,
which articulates with a portion of two bones, the lacrimal anteriorly,
and the os planum of the ethmoid bone posteriorly. Only the posterior
two-thirds of this border presents an articulating edge, the remaining or
anterior third being smooth and forming the commencement of the lacri-
mal groove, which in the articulated skull becomes a canal, passing
downward and backward to communicate with the inferior meatus of
the nose. Beginning at the posterior border of this surface and running
forward will be found a deep groove — the infra-orbital groove. When
near the center of the surface, this groove dips down and is covered by a
layer of bone, from which point it passes forward as a canal — the infra-
orbital canal — making its exit at a point about \ of an inch below
the border of the orbit, near the center of the facial surface of the
bone, the foramen thus formed being the infra-orbital foramen. Near
the root of the nasal process, and immediately within the anterior border
of this surface, is a small depression which marks the origin of the in-
ferior oblique muscle of the eyeball.

The Lateral or Facial Surface (Fig. 15). — This surface is made
up of the anterior part of the bone; it is irregularly concave, and pre-
sents a greater variety in form than any other part of the bone, with the
single exception of the palatal process. It is bounded above by the infra-
orbital ridge, and the roughened surface of the malar process which ar-
ticulates with the malar bone; below, by the border of the alveolar process;
anteriorly, by the frail concave border of the opening into the nasal cavity,
the anterior nasal spine, and the perpendicular margins of the bone be-
neath. Posteriorly, this surface is separated from the posterior or zygo-
matic surface by a strong projecting eminence, the malar process.

The canine fossa is a deep depression, situated almost in the center
of this surface, the bone at this point being extremely thin and closely re-
lated to the floor of the antrum. The concave floor of this fossa is fre-
quently traversed by one or two smaller convex ridges, corresponding to
the roots of the bicuspid teeth.

The canine eminence is a prominent ridge running vertical to the
body of the bone immediately anterior to the canine fossa, and corre-
sponding in position to the root of the cuspid tooth, the size and type of
the tooth having much to do with its extent and prominence. This
ridge gives origin to one of the depressor muscles of the upper lip, and also

ANATOMY.

to one of the depressor muscles of the wing of the nose. The incisive or
myrtiform fossa is a depression found between the canine eminence and
the inner margin of the bone. The depth of this fossa is in a measure
controlled by the position and size of the teeth, and by the amount of
prominence in the canine eminence. j"*

The infra-orbital foramen, which transmits the infra-orbital nerves
and blood-vessels, is immediately below the center of the infra-orbital

Nasal Process

Infra-orbital
Foramen

Canine Fossa

Nasal Spine
Facial Suiface

Incisive Fossa

Canine
Eminence

Orbital Surface

Zygomatic
Surface

Malar Process

Posterior Dental
Canals

Tuberosity

Fig. 15. — Left Superior Maxilla, Outer or Facial Surface.

ridge, and near the upper margin of the canine fossa. It is oval in form,
and faces almost directly toward the median line. Between this foramen
and the infra-orbital ridge is the point of origin for the principal levator
muscle of the upper lip, the levator labii superioris proprius. The whole
extent of the facial surface may present a number of vertical ridges, or the
same space may be regular and smooth, the condition being controlled
by the size and shape of the tooth-roots and the thickness of the bone
covering them. One of the levator muscles of the angle of the mouth,
the levator anguli oris, is attached to this surface near the upper border
of the canine fossa.

SUPERIOR MAXILLARY BONES.

The Proximal or Nasal Surface (Fig. 16). — Above, this surface
presents a large, irregular opening into the maxillary sinus, this opening
being nearly closed in the articulated skull by neighboring bones. In
front of the opening into the sinus, and standing perpendicular from the
body of the bone, is the strong ascending plate of the nasal process,
marked near its lower extremity by a rough, horizontal ridge, the inferior

Antrum of
Highmore

Inferior Meatus

Posterior Pala
tine Groovt

Palate Proces>

Nasal Process

Ridge for Mid-
dle Tuibinal

Middle Meatus

Thin plate of
bone over Lac-
rymal Groove

Ridge for Infe-
rior Turbinal

Nasal Spine

Anterior Pala-
tine Groove

Fig. 16. — Left Superior Maxilla, Internal, Proximal, or Nasal Surface.

turbinated crest, which gives attachment to the inferior turbinated bone.
The smooth, concave surface immediately above this ridge corresponds
to the middle meatus of the nose, and forms the external wall of that pas-
sage. Below the opening into the sinus and the nasal process, and occupy-
ing the anterior two-thirds of the middle of this surface, is a large semi-
circular space, forming the outer wall of the inferior meatus of the nose.
Below this space, and projecting inward from the body of the bone, is
the palatal process, which articulates with the corresponding process

38 ANATOMY.

of the opposite bone. At the anterior superior angle of the nasal surface,
and passing downward just behind the nasal process, is the lacrimal
groove. In the articulated skull this groove becomes a canal, the lacri-
mal canal, the ethmoid and the inferior turbinated bones assisting in its
formation. The canal passes downward and slightly backward, and
opens into the inferior meatus of the nose. It is about 1/2 of an inch in
length, and gives passage to the lacrimonasal duct.

The lacrimal tubercle is a small prominence of bone formed at the
junction of the anterior border of this surface, with the external surface
of the nasal spine. The extended portion of the lacrimal duct, the
lacrimal sac, finds lodgment at this point.

The posterior palatine or palatomaxillary canal commences near the
middle of the posterior border of this surface, appearing in the disartic-
ulated bone as a groove, and, passing downward and forward, gives pas-
sage to the posterior palatine vessels and anterior palatine nerves. The
canal is made complete by the articulation of the superior maxillary with
the vertical plate of the palate bone. On the posterior portion of the nasal
surface, extending from the irregular opening into the antrum downward
to a point opposite the palatal process, is a roughened surface about 1/2
of an inch in width, which marks the extent of articulation with the
palate bone.

The proximal or nasal surface presents four borders — superior, in-
ferior, anterior, and posterior. The superior border is irregular, and
articulates with the lacrimal and ethmoid bones. The inferior border
projects inward, and forms a strong horizontal plate — the palatal process.
This process defines the border from before backward to the posterior
third, at which point it is marked by the lower border of the roughened
surface which articulates with the palate bone. The anterior border is
sharp, frail, and irregular in outline, and forms the free margin of the
opening into the nasal cavity. The posterior border is marked by the
inner margin of the zygomatic surface, being smooth upon its upper half,
and roughened upon its lower half, at which point it articulates with the
palate bone.

The Posterior or Zygomatic Surface (Fig. 16). — This surface is
partly convex and partly concave, and is bounded above by a well-de-
fined margin, which serves as the dividing-line between this and the
superior or orbital surface. This border is also marked by a roughened
margin on the posterior portion of the malar process, the orbital portion
of the palate bone articulating at this point. The major portion of this
border is smooth and rounded, forming the lower border of the spheno-

SUPERIOR MAXILLARY BONES. 39

maxillary fissure, and marked by a notch, the commencement of the infra-
orbital groove. The outer border of the surface is formed by the malar
process, and by a line drawn from this point directly downward to the
alveolar process. The inner border is smooth and somewhat irregular
above, while below it is roughened for articulation with the palate bone.

The tuberosity, which also forms a portion of the inferior border of
this surface, is a roughened and rounded eminence of bone, and is pene-
trated by a number of nutrient vessels, which enter the many small fora-
mina at this point. Between the tuberosity and the body of the zygomatic
surface are several large apertures leading into canals, which pass into,
and give nourishment to, the substance of the bone. These canals trans-
mit the posterior dental blood-vessels and nerves, one of which, after
passing over the outer wall of the maxillary sinus, unites with the anterior
dental canal. The tuberosity is posterior to, and above, the third molar
tooth, in some instances extending directly backward from this tooth for
the distance of half an inch or more, but usually the tooth penetrates the
base of the tuberosity, leaving but a thin layer of bone posterior to it.

The inferior border of the posterior or zygomatic surface is formed
by that portion of the alveolar process which supports the second and
third molar teeth.

The bone presents four processes for examination — the nasal, the
malar, the palatal, and the alveolar.

The nasal process is a strong, irregular piece of bone, standing ver-
tically above the body of the bone proper, and forming the lateral boundary
of the nose. This process is greatly increased in strength by the infra-
orbital ridge joining it at or near its base, and ascending its external
anterior surface to some extent. That portion of the process posterior
to its junction with the infra-orbital ridge assists in forming the inner
wall of the orbit.

The external or anterior surface of the nasal process is marked by a
number of shallow grooves, traces of the development of the bone.
Scattered over this surface are a number of small foramina, the entrances
to minute canals transmitting nutrient vessels to the body of the bone.
This surface gives origin to one of the lip muscles, the levator labii superi-
oris alaeque nasi.

The internal surface of the nasal process is usually described as in-
cluding all that portion between the superior border and the floor of the
anterior nares. The surface is marked by two concave portions and
two ridges. The two ridges divide the surface into three parts — the
superior meatus, the middle meatus, and the inferior meatus of the nose.

40 ANATOMY.

The superior meatus is the smallest of the three, and occupies the slightly
concave space above the superior ridge. The middle meatus, partly
concave, and partly convex, includes the space between the superior
and the inferior ridges, and extends from the free margin of the bone in
front to the lacrimal groove behind. The inferior meatus, which is
much the largest, occupies all that concave surface between the inferior
ridge above and the palatal process below, and extends from the anterior
margin of the bone backward to the point of articulation with the palate
bone. The two ridges previously referred to are known as the superior
turbinated crest, which articulates with the middle turbinated bone, and
the inferior turbinated crest, which articulates with the inferior turbinated
bone.

The malar process is a large, irregular portion of bone situated
at the angle of separation between the facial and zygomatic surfaces,
and presents a triangular, roughened surface for articulation with the
malar bone. The superior boundary of this process is formed by the
orbital surface and the outer end of the infra-orbital ridge; the inferior
boundary may be marked by an irregular imaginary line running from
the upper margin of the canine fossa to a point between the first and second
molar teeth, while the posterior inferior boundary may be traced from
the outer superior angle of the zygomatic surface downward and forward
to the point above referred to. This process, as well as the nasal process,
is subject to much variety in form and general outline. The malar
process, assisting as it does in forming what is commonly called the cheek
bone, is particularly variable in size, and in certain types and races it is
so prominent as to become a controlling feature in the facial form. One
of the muscles of mastication — the masseter — has a portion of its origin
from the malar process.

The palatal process is more directly interested in the formation
of the cavity of the mouth than any other portion of the superior maxillary
bone. By articulating with its fellow of the opposite side, it forms about
three-fourths of the hard palate or roof of the mouth, the remaining fourth
being formed by a portion of the palate bones. It is thick and strong, and
projects horizontally inward from the inner surface of the body of the
bone. It presents two surfaces for examination — a superior or nasal
surface, and an inferior or oral surface. The superior or nasal surface
is smooth and more or less concave, and forms the floor of the nares.
The inferior or oral surface is also concave, but is much roughened by
numerous small projections, between which are lodged the mucous
glands. Upon the anterior portion of this surface are a number of

SUPERIOR MAXILLARY BONES. 4 1

small foramina, which mark the entrance to numerous small canals
giving passage to nutrient vessels to supply the body of the bone. Near
the center of the posterior third are the anteroposterior grooves, which
accommodate the posterior palatine nerves and blood-vessels. This
process also presents for examination three borders and various other
points of interest. The three borders are the interior,' posterior, and
mesial. The anterior border is thick and somewhat irregular; the posterior
border is thin and frail, and articulates with a portion of the palate
bone. The mesial border presents a wide articulating surface in front,
behind it is narrow, the whole extent of this border articulating with the
corresponding process of the opposite bone.

The Nasal Spine. — At the anterior superior angle of the palatal
process is a well-defined spine — the nasal spine — being formed by a pro-
longation of the process beyond the level of the facial surface of the bone.
This process, when articulated with its fellow of the oposite side, forms
the base of the nose.

The Nasal Crest. — Beginning at the base of the nasal spine, and ex-
tending backward along the median border of the bone, is a sharp, irre-
gular ledge of bone, the nasal crest. This portion of the process ar-
ticulates with the vomer.

The incisor crest is a continuation of the nasal crest anteriorly, pro-
jecting beyond the nasal spine in the form of a sharp, spear-like point.

The incisive foramen, ox foramen of S tens on, is situated immediately
back of the incisor crest, and leads downward and forward from the nasal
chamber toward the mouth, entering that cavity just back of the central
incisor tooth. This passage in the single bone is a simple groove, but
in the articulated skull it becomes the anterior palatine canal, which,
after passing downward, opens on the nasal surface of the palatal process
by four foramina — the incisive foramina, and the foramina of Scarpa, or
the naso-palatine foramina. These foramina transmit the naso-palatine
nerves.

The palatal process of the superior maxilla is subject to a greater
variety in form than any other portion of the bone, this variation in the
articulated skull being the cause of the many different curves assumed
by the roof or dome of the mouth.

The Alveolar Process. — This process forms the lower margin of
the bone, and extends from the base of the tuberosity behind to the
median line in front, at which point it articulates with the same process
of the opposite bone. It has an outer and an inner margin corresponding
to the buccal and palatal surfaces of the roots of the teeth, which are

42 ANATOMY.

firmly imbedded in it. Its general form from before backward is that of
a gradual curve, somewhat variable in different bones, the extent of this
variation depending on the type or race to which the bone belongs. The
body of the process is made up of an outer and an inner plate, which are
connected by numerous septa of cancellated bone. The outer plate of
the process is continuous with the facial and zygomatic surface of the body
of the bone, and assists in forming these surfaces. It is quite thin and
frail, and the position of the alveoli beneath are well shown by the numer-
ous vertical ridges upon it. The inferior margin of the outer plate is
reinforced by an additional thickness of bone, forming the border of the
alveolar sockets. That portion of the plate supporting the molar teeth
is heavier in general, and the position of the sockets are not so plainly
outlined. The inner plate of the alveolar process is much heavier and
stronger than the outer plate, and extends from the margins of the alveoli
below to the palatal process above. The inferior margin of this plate is
not reinforced except in the region of the molars. The construction of
the inner plate is, to a great degree, controlled by the shape and position
of the palatal process. In the lymphatic temperament this process,
when articulated with its fellow, forms a flat or shallow dome to the oral
cavity, and in so doing gradually curves into the alveolar process, giving
it additional thickness. The depth of the process in this type is not
great, and the roots of the teeth are short and heavy in proportion. In
the bilious temperament the inner alveolar plate is deep and abrupt, ex-
tending from the inferior margin upward in almost a perpendicular direc-
tion to the palatal process which joins it almost at right angles. The
alveolar process gives origin to one of the cheek muscles — the buccinator
— which is attached to the outer plate near its upper margin, and directly
over the space occupied by the second bicuspid and first molar teeth.
The Alveoli or Tooth Sockets. — These cavities, which are variable
in number, are formed by the outer and inner plate of the alveolar process,
and by numerous connecting septa of bone placed between the two plates.
The shape and depth of each cavity is regulated by the form and length
of the roots of the teeth which they support. The first socket, or that
next to the median line, gives support to the central incisor tooth. It
forms almost a perfect cone, and has an average depth of half an inch.
Its lower border is circular, and the anterior or labial portion describes
a larger circle than the posterior or palatal half. The mesial and distal
walls are somewhat flattened. The second cavity, proceeding backward
from the median line, supports the lateral incisor tooth. It is also conic,
but somewhat smaller than the preceding. It is seldom over 3/8 to 5/16

SUPERIOR MAXILLARY BONES. 43

of an inch in depth. It is much flattened on its mesial and distal walls,
giving the appearance of an oblong, rather than a round, cavity in trans-
verse section. This socket, as well as that for the central incisor, occupies
an almost vertical position in the process. Very frequently the socket
for the lateral incisor presents a slight distal curve at its upper extremity.
The third socket, or that giving support to the cuspid tooth, is much
larger and deeper than either of those previously described. It extends
upward, inward, and backward with an average depth of 5/8 to 3/4 of
an inch. In transverse section, its labial wall presents a much larger
circle than its palatal margin. The labial and distal walls are much
flattened and somewhat convex. The general direction of this socket is
slightly to the distal. The socket which supports the first bicuspid is
usually divided from mesial to distal by a thin septum of bone, thus
forming an outer or buccal socket, and an inner or palatal socket. This
division seldom exists to the full depth of the cavity, but usually begins
about midway of its length. The lower margin of this socket is oblong or
egg-shaped, its outer or buccal portion forming a larger curve than its
palatal. The lateral walls are slightly concave or flattened, until the
point of separation is reached, when they become more circular, the alveoli
above this point becoming cone-shaped. It is not uncommon for this
socket to be a single cavity, and when thus formed it resembles a flattened
cone, with the buccal and palatal margins rounded. The next socket
gives support to the second bicuspid tooth, in most instances being a
single cavity, but in rare instances it is divided near its upper extremity.
In general outline it resembles the socket for the first bicuspid.

The socket for the first molar is much larger than any of those
previously described; its inferior margin presents a circular outline on its
buccal and palatal portions, the former curve being larger than the latter.
The mesial and distal walls are flattened and slightly concave. The
upper three-fourths of this socket is divided into three separate compart-
ments, being so arranged that two are upon the buccal and one upon the
palatal side. The septa separating the two buccal cavities from the pala-
tal cavity are heavy and strong, while that placed between the two buccal
sockets is thin and frail. The two buccal cavities are usually flattened
upon their mesial and distal sides. The palatal socket is larger and
somewhat deeper than the buccal, the average depth of all being about
1 1 2 of an inch. The socket for the second molar is similar in most respects
to that for the first molar, except that it is somewhat smaller. The same
description might answer for the third molar socket, which in general
is similar to the alveoli for the other molars. It is smaller than the second

44 ANATOMY.

molar socket, and may be a single cavity, or it may be divided into three
compartments.

Articulations. — The superior maxillary bone articulates with
its fellow of the opposite side, with the frontal, lacrimal, ethmoid, palate,
vomer, malar, and inferior turbinated bones. Occasionally it articulates
with the sphenoid bone.

Attachment of Muscles. — The muscles attached to this bone are
eleven in number, and are as follows:

Compressor nares, Internal pterygoid,

Orbicularis oris, Orbicularis palpebrarum,

Levator labii superioris akeque Levator labii superioris proprius,

nasi, Inferior oblique,

Levator anguli oris, Buccinator,.

Depressor ate nasi, Masseter.

Blood-supply. — The maxilla receives its vascular supply from
numerous large arteries. They are derived from the alveolar, infra-
orbital, nasopalatal, descending palatal, ethmoidal, nasal, frontal, and
facial branches.

Development. — The superior maxilla arises from four points of
ossification, which are deposited in membrane. These four centers
make their appearance as early as the eighth fetal week, this early begin-
ning making it somewhat difficult to accurately follow its growth. The
four centers are named, as located, premaxillary, maxillary, malar, and
prepalatal. The premaxillary nucleus gives rise to the incisive portion
of the bone, or that part supporting the incisor teeth. During early life
this division of the bone is separated from the body of the bone, and is
known as the premaxillary portion (Fig. 17). Union between the pre-
maxillary portion and the maxilla proper takes place about birth, and the
suture thus formed is visible on the facial surface until the sixth or seventh
year, and on the palatine surface until the adult period. The palatal
suture extends as far back as the posterior border of the anterior palatal
canal. This nucleus also sends a narrow process upward which forms
part of the outer boundary of the anterior narial aperture. On the
palatal aspect it furnishes a speculum which surrounds the anterior and
mesial walls of Stenson's canal. The posterior limit of the premaxillary
portion is indicated by the suture on the palatal surface. The maxillary
nucleus forms the greater portion of the body of the true maxilla and the
nasal process. The malar center gives origin to the malar process,
and all that portion external to the infra-orbital groove. The prepalatine

3RARY

SUPERIOR MAXILLARY BONES. 4S

W.v.u,

center gives rise to the nasal surface of the bone and that portion of the
palatal process posterior to Stenson's canal.

Development of the Alveolar Process. — This process is represented
at birth by the walls of a deep groove, in which are lodged the partly
calcified, deciduous teeth and the germs of most of the permanent teeth
(see Development of the Teeth).

The growth of the process continues with the growth of the teeth
until, finally, at about the seventh month after birth, the dental organs

Premaxillan
Portioi

Suture

Palatal Portion

Malar Portion

Fig. 17. — Left Superior Maxillary, about the Third Year, Enlarged.

are completely encased within its walls. With the decalcification of the
roots of the deciduous teeth comes the loss of the process surrounding
them, and, as the permanent teeth advance to take their place in the
arch, the process is again built up about their roots.

The Maxillary Sinus, or Antrum of Highmore* (Fig. 16). — This
is a large cavity situated within the body of the maxilla. Its general
shape is that of a pyramid, with its base directed toward the median line,
or nasal surface, its apex pointing toward and extending into the malar
process, and, in some instances, penetrating the malar bone. The size
of the cavity varies in different subjects and in the opposite bone of

* Described separately, in preference to including in general description of the bone.

ANATOMY.

the same subject. The average capacity is about three fluidrams, but
this may be increased to six or eight fluidrams. The size of the bone and
the prominence of the malar process control, in a measure, the size of
the cavity; but not infrequently the largest bone will present the smallest
sinus. Sex also appears to exert a controlling influence over the capacity
of the cavity, it being greater in the male than in the female. In youth
the cavity is quite small, the walls being much thicker proportionately
than in the adult. The walls of the sinus in the matured subject are
quite thin and frail, and are four in number. The superior wall is formed

FlG. 18. — Developing Maxillary Bones about the Fifth Month after Birth.

by a thin plate of bone, the floor of the orbit. This surface is almost flat,
and serves as a roof to the cavity. Near the anterior margin of this sur-
face is a thick rib of bone which marks the course, and forms one of the
walls of the infra-orbital canal.

The inner wall, or that looking toward the nasal surface, is formed
by the thin bony layer separating this cavity from that of the nares. The
outer or lateral surface, formed by the facial and zygomatic surfaces of
the bone, is smooth, and convex from before backward. Near the center
of this surface the cavity may penetrate the malar process, and in the dis-
articulated skull would present an opening at this point. The inferior
wall is formed by the alveolar process, and is marked by a number of
irregular eminences corresponding to the roots of the neighboring teeth.
The teeth referred to are generally the first and second molars, and occa-

THE PALATE BONE. 47

sionally the second bicuspid. It is not unusual for the roots of one or more
of these teeth to penetrate the floor of the sinus, in consequence of which the
lining membrane of the cavity may suffer disease generated in the teeth.

The inferior wall is much the strongest of the four, and, besides the
unevenness of the surface produced by the tooth-roots, it frequently
supports a number of thin, bony partitions, which may completely or
partly divide the floor of the cavity into numerous small compartments.

The posterior portion of the lateral wall is marked by the posterior
dental canals, which give passage to the posterior nerves and blood-vessels.
In like manner the anterior portion of the lateral wall is grooved for the
reception of the anterior dental nerves and blood-vessels. Upon the inner
wall, or that forming the base of the pyramid, is an opening which commu-
nicates with the middle meatus of the nose. In the articulated skull this
opening is quite small, being from 1/8 to 1/4 of an inch in diameter.
The correct idea of this opening can not be obtained by studying the
individual bone, as the numerous perforations then to be observed are
closed or partly closed by articulation with adjacent bones. The mucous
membrane lining the nasal cavity enters the sinus through the small
aperture above referred to, and forms a continuous lining over its entire
surface. Cryer has thrown much light upon the relations of the maxillary
sinus to the mouth and teeth, and he has demonstrated beyond a doubt
that the relationship existing between the parts is susceptible to extensive
variation. He has shown that in some instances the cavity upon one
side will be large, with its floor broken by the tooth-roots, while that upon
the opposite side will be extremely small and far removed from the root
apices. In fact, these researches have so revolutionized .the subject
under consideration that the foregoing description is only reliable in so
far as it treats of the conditions most frequently met with.

THE PALATE BONE.

The palate bones (Fig. 19), two in number, are situated immediately
posterior to the two maxillae, and with them complete the hard palate.
They also assist in forming the boundaries of the orbital and nasal cavities,
the sphenomaxillary, the sphenopalatine, and the pterygoid fossa, the
sphenomaxillary fissure, the posterior ethmoidal cells, and the maxillary
sinus. When in position in the skull, these bones are wedged between
the maxillae and the sphenoid bone. They are rectangular in outline, and
each bone presents for examination a horizontal and a vertical plate, a
tuberosity, and two processes, the orbital and the sphenoid.

ANATOMY.

The horizontal plate, smaller than the vertical, assists in forming
the hard palate, and corresponds to the palatal process of the maxilla.
In entering into the construction of the hard palate the form of this plate
varies to the same degree as the palatal plate of the maxilla. In general,
it is described as quadrilateral in shape, having two surfaces and four
borders. The superior surface, which is concave from side to side, forms
the posterior floor of the nasal chamber. The inferior surface completes
the hard palate posteriorly, and presents, near its posterior border, a
transverse ridge for the attachment of one of the muscles of the soft
palate (the tensor palati) ; the anterior border is serrated for articulation

*CRjOSITV
PTERYGOID FOSSA

Fig. 19. — The Two Palate Bones in their Natural Position,
Dorsal View. (Testut.)

with the palatal process of the maxilla. The posterior border is free,
curved, and sharp, and marks the posterior boundary of the hard palate.
At the median line this border terminates in a sharp point, which, when
articulated with the corresponding bone of the opposite side, forms the
posterior nasal spine; to this point the azygos uvula* muscle is attached.
The external border is situated just below the junction of the horizontal
and vertical plates. In this portion is a groove which assists in forming
a portion of the posterior palatal canal. The internal border is broad
and serrated for articulation with its fellow of the opposite side. When
the palate bones are in position in the skull, these borders form a ridge, con-
tinuing the crest formed by the palatal process of the maxilla, this crest
receiving the inferior border of the vomer.

The vertical plate is thin and frail and extends from the floor of
the nasal chamber below to the upper extremity of the sphenopalatine
notch above. It has two surfaces and four borders.

THE PALATE BONE. 49

The external surface is roughened for articulation with the maxilla,
excepting a small triangular surface near the upper extremity, which
forms a portion of the sphenomaxillary fossa, and a small portion near
the middle of the surface close to the anterior border, which forms a
portion of the wall of the maxillary sinus. Near the posterior boundary
of this surface is a vertical groove, which forms, when articulated with the
maxilla, the posterior palatal canal, transmitting the descending palatal
nerves and vessels.

The internal surface is divided into three shallow depressions by two
transverse ridges — the superior and inferior turbinated crests. The
lower depression thus formed assists in the construction of a portion of
the interior meatus of the nose. The crest immediately above this depres-
sion articulates with the inferior turbinated bone. The central depres-
sion, the largest of the three, forms a portion of the middle meatus of the
nose, the crest above articulating with the middle turbinated bone. The
superior depression — much smaller but deeper than either of those pre-
viously described — forms a large part of the superior meatus. The an-
terior border of the vertical plate is thin and sharp, the inferior turbinated
crest protruding near the center of the border, and forming the maxillary
process. This process assists in closing the maxillary sinus by being
received into the maxillary fissure of the maxilla. At the upper extrem-
ity of this border is the orbital process, which presents for examination
five surfaces, three of which are articular. The anterior or maxillary
surface is directed outward, upward, and downward. It is oblong in
form and articulates with the posterior superior angle of the inner surface
of the maxilla. The posterior or sphenoidal surface is directed back-
ward, upward, and inward, and articulates with the vertical plate of the
ethmoid bone. The superior or orbital surface is triangular in form,
extending upward and outward, forming the posterior angle of the floor
of the orbit. The external or zygomatic surface is smooth, oblong, and
directed outward, backward, and downward, forming a portion of the
sphenomaxillary fossa.

The posterior border of the vertical plate is irregular and serrated,
and comes into relation with the internal pterygoid process, terminating
below in a prominent tuberosity. This presents three grooves or flutes.
The inner receives the internal pterygoid, the outer the external pterygoid
process, while the middle groove completes the pterygoid fossa, and
gives attachment to a portion of the internal pterygoid muscle. This
process also gives rise to the superior constrictor of the pharynx.
Passing through the tuberosity are a number of small canals, those on the

50 ANATOMY.

nasal side being the accessory palatal canals. Near the junction of the
tuberosity with the horizontal plate is the opening of the posterior palatal
canal, and beyond this the small external palatal canals.

The sphenoidal process is at the superior end of the posterior border.
It is variable in shape and curves upward, backward, and inward. It
presents a superior, an external and an internal surface, and two borders
— an anterior and a posterior.

The superior surface, the smallest of the three, is marked by a groove,
which assists in forming the sphenopalatine canal. This surface articu-
lates with the horizontal portion of the sphenoidal turbinated bone.
The external surface assists in forming the sphenomaxillary fossa by its
anterior portion, while the posterior portion is rough for articulation
with the pterygoid plate of the ethmoid bone. The internal surface is in-
strumental in forming a portion of the outer wall of the posterior nares,
and for this purpose is smooth and concave. The anterior border
forms the posterior margin of the sphenopalatine notch. The posterior
border is serrated, and articulates with the inner surface of the pterygoid
process.

The superior border of the vertical plate is divided by a deep notch or
foramen, which divides the orbital from the sphenoidal process. This
opening is the sphenopalatine notch or foramen, and transmits the spheno-
palatine vessels and nerves from the sphenopalatine fossa to the nasal
chamber.

The inferior border of the vertical plate joins the external border of
the horizontal plate. Extending downward and backward from the
inferior and posterior borders is the pyramidal process, the borders
of which are serrated for articulation with both pterygoid plates of the
sphenoid bone.

Articulations. — The palate bone articulates with the sphenoid,
superior maxilla, sphenoidal turbinated, inferior turbinated, ethmoid,
and with its fellow of the opposite side.

Attachment of Muscles. — The following muscles are attached to
the palate bone:

Tensor palati, Internal pterygoid,

Azygos uvulae, Superior constrictor of pharynx.

Blood-supply. — The arteries which supply this bone are derived
from branches of the descending palatine, the sphenopalatine, and
pterygopalatine.

Development. — The palate bone is developed from a single center

INFERIOR MAXILLARY BONE.

deposited in membrane. This center makes its appearance about the
eighth or ninth fetal week, near the line of junction between the horizon-
tal and vertical plates. At birth these plates are about the same length,
but soon after this period, when the nasal sinuses increase in height,
the vertical plate begins to lengthen, and continues to do so until it be-
comes nearly double the length of the horizontal plate.

INFERIOR MAXILLARY BONE.

The Inferior Maxillary, Mandible, or Lower Jaw Bone (Fig. 20). —
This bone, having no osseous union with the skull proper, may be con-

Fig. 20. — The Mandible or Inferior Maxilla. Right Side, External or Facial Surface.
a, Sigmoid Notch; b, Coronoid Process; c, Condyle; d, Neck of Condyle; e, Ramus;
/, External Oblique Line; g, Angle; h, Mental Foramen; i, Mental Protuberance; j, Body.

sidered as one of its appendicular elements. It is the heaviest and
strongest bone of the head, gives support to the sixteen lower teeth, and
serves as a framework for the lower half or floor of the mouth. It is
situated at the lower extremity of the face, and immediately below the
superior maxillary and malar bones, while its posterior extremity rests

52 ANATOMY.

against the glenoid fossa of the temporal bone, forming a movable articu-
lation with this cavity. In general, the bone is symmetric in outline,
and presents for examination a horizontal portion, or body, and two
vertical portions, or rami, which in the adult are almost perpendicular to,
or at right angles with, the body of the bone.

The body, or horizontal portion, consists of two identical halves,
which meet at the median line and form a slight vertical ridge, the sym-
physis. This line indicates the point of union between the two lateral
halves, which at birth are usually separated, but soon after this period
become firmly united. Each lateral half of the body presents two sur-
faces — an external and an internal; and two borders — a superior and an
inferior.

The external or facial surface (Fig. 20) is smooth and convex,
and furnishes a number of points for examination. Beginning at the
median line, the symphysis ends inferiorly in a prominent triangular
surface — the mental protuberance, or chin.

The Incisive Fossa. — Passing backward from the symphysis, and
immediately above the triangular ridge which forms the mental process,
is a decided but shallow depression — the incisive fossa. This fossa
gives origin to one of the elevator muscles of the chin — the levator menti.
Slightly posterior to and below this fossa, on a line corresponding to the
position of the cuspid tooth, is an oblong depression for the origin of the
depressor muscle of the lower lip — depressor labii inferioris.

The External Oblique Line. — Extending obliquely across the facial
surface from the mental process to the base of the vertical portion of the
bone, and continuous with its anterior margin, is a well-defined ridge—
the external oblique line. Near the center of this ridge, or below the
position occupied by the bicuspid and first molar teeth, is the point of
attachment of the depressor muscle of the angle of the mouth — the de-
pressor anguli oris. Somewhat anterior to and above this point is the
origin of the depressor muscle of the lower lip — the depressor labii inferi-
oris. Between the line of origin of the depressor anguli oris and the in-
ferior border of the bone is a roughened surface for the attachment of
the platysma myoides muscle. This roughened surface divides the body
of the bone into an upper and a lower portion. That portion above is
known as the alveolar or mucous portion, while that below is called the
basilar or non-mucous portion. The attachment of the platysma myoides
muscle at this point marks the lower boundary or floor of the mouth. The
superior or alveolar portion of the bone is within the cavity of the mouth,
and is covered with mucous membrane and mucoperiosteum; while the

INFERIOR MAXILLARY BONE. 53

inferior or basilar portion is outside and below the cavity, and is covered
with periosteum similar to other bones.

The Mental, or Anterior Dental Foramen. — Midway between the
superior and inferior border of the body, and usually below the second
bicuspid tooth, is a large foramen — the mental or anterior dental fora-
men — giving passage to the mental branches of the inferior dental nerve
and accompanying blood-vessels. The position of this foramen is not
constant, but, as previously stated, it is usually below the second bicuspid,
or between this point and the first bicuspid. The buccinator muscle,
which forms a large portion of the lateral wall of the mouth, has its origin
from the facial surface of the mandible, being attached to the alveolar
portion immediately below the molar teeth.

The Internal Surface of the Body of the Bone (Fig. 21). — The
median line is marked by a slight vertical depression, representing the line
of union, and corresponding to the symphysis externally.

The Mylohyoid, or Internal Oblique Ridge. — The internal surface is
divided into two portions by a well-defined ridge — the mylohyoid, or
internal oblique ridge. It occupies a position closely corresponding to the
external oblique ridge on the facial surface. Beginning near the base
of the bone at the median line, it passes backward and upward, increasing
in prominence until the base of the vertical portion of the bone is reached,
into which it gradually disappears. This ridge gives origin to the mylo-
hyoid muscle, which forms the central portion of the floor of the mouth.
In correspondence to the facial surface of the bone, the attachment of the
mylohyoideus muscle forms the dividing line between the mucous mem-
brane and mucoperiosteum covering the upper portion of the body of
the bone, and the periosteum covering the inferior portion.

The Genial Tubercles. — Near the lower third, at the median line, is a
roughened eminence — the genial tubercles. Taken collectively, these are
in two pairs — a superior and an inferior. The superior pair (usually the
largest) give origin to the geniohyoglossus muscle, and the lower pair to
the geniohyoid muscle.

The Sublingual Fossa. — By the side of the genial tubercles, and
above the mylohyoid ridge, is a shallow, smooth depression — the sub-
lingual fossa. One of the salivary glands — the sublingual — is partially
supported in this fossa.

The Digastric Fossa. — Below the mylohyoid ridge, and near the
median line, is a slight depression — the digastric fossa — which affords
attachment for the digastric muscle.

The Submaxillary Fossa. — In the center of the internal surface, ex-

ANATOMY.

tending from before backward, between the mylohyoid ridge and the
lower border of the bone, is an oblong depression — the submaxillary
fossa. In this fossa rests another of the salivary glands — the maxillarv.
The Superior or Alveolar Border. — This border extends from
the junction of the body, with the vertical plate on one side, to the corre-
sponding point on the other. The construction of this border is similar
to the alveolar border of the superior maxilla. At the anterior portion

a b

Fig. 21. — The Mandible or Inferior Maxilla. Right side, Internal Surface, a, Cofonoid
Process; t>, Sigmoid Notch; c, Cancellated Tissue; d, Condyle; e, Ramus; /, Inferior
Dental or Mandibular Foramen; g, Angle; h, Body; i, Internal Oblique Line.

it is narrow, but gradually increases in width as it proceeds backward—
in some instances following the line of the body of the bone; in others,
inclining inward, or to the lingual. Each lateral half is marked by eight
sockets, for the accommodation of the sixteen lower teeth. They arc-
smaller in proportion than the alveolar sockets in the superior maxilla.
The socket nearest the median line receives the central incisor tooth,
and is the smallest of the number. It has an average depth of 7/16 of
an inch, is conic from above downward, oblong in transverse section,
with its lateral walls flattened. The second socket gives support to the
lateral incisor tooth; it is a trifle larger than the central incisor socket,

INFERIOR MAXILLARY BONE. 55

but in other respects is quite similar. The socket for the cuspid
tooth is situated at the anterior angle of this border, and is much
larger and deeper than the incisor sockets. It has an average depth of
9/16 of an inch; its lateral walls are compressed, and sometimes slightly
concave. In transverse section the labial wall forms a larger curve than
the internal or lingual wall. Passing backward, the next two sockets are
for the support of the bicuspids; they are circular in outline, with an aver-
age depth of 1/2 of an inch. The cavity for the first bicuspid is usually
a little larger than that for the second. In rare instances one or the other
of these sockets will be divided for the accommodation of two roots.
The sockets for the first and second molars present a circular outline
upon their free margins, but below they divide into two flattened, cone-
shaped cavities — one anterior and one posterior. The flattened sides of
these cavities are concave in the center, and at their lower third curve
backward. The average depth of these sockets is 1/2 of an inch. The
socket for the third molar, like its superior fellow, is variable both in form
and position, frequently being crowded inside or outside of the tooth-line.
In some instances it is divided into two or more compartments. The
average depth is not over 3/8 of an inch.

The alveolar process, which composes the superior border of the
body of the mandible, differs from the same process in the superior max-
illa in one very important particular: instead of the outer plate being
thin and frail, it is equally as heavy as the inner or lingual plate. When
the tooth-line is inclined inward from the body of the bone, the posterior
outer wall is much heavier than the interior.

The Inferior Border of the Body of the Bone. — This border extends
from a slight depression, to be observed at the point of union between
the body and ramus, to the corresponding point upon the opposite side.
It is strong, rounded, and compact, and gives to the bone the greatest
portion of its strength. Near its junction with the ramus is the facial
notch, so named from the facial artery passing over this point.

The Ramus, or Vertical Portion of the Bone. — This vertical
plate is quadrilateral in outline, and presents two surfaces — external
and internal; four borders — superior, inferior, anterior, and posterior;
and two processes — the condyloid and the coronoid.

The external surface is flat and smooth. Near the center it is slightly
concave and roughened for the attachment of one of the muscles of mas-
tication — the masseter.

The internal surface presents near the center an oblong opening —
the inferior dental or mandibular foramen — leading into the inferior dental

56 ANATOMY.

or mandibular canal. Surrounding this foramen, on its posterior inter-
nal margin, is the mandibular spine, to which is attached the spheno-
mandibular ligament. Running obliquely downward from the base of
the foramen, and beneath the spine, is a decided groove — the mylohyoid
groove — which accommodates the mylohyoid nerve, artery, and vein,
which pass forward to supply the floor of the mouth. Below and behind
this groove the surface is roughened for the attachment of another muscle
of mastication — the internal pterygoid.

The Inferior Dental or Mandibular Canal. — Beginning at the foramen
of the same name, this canal enters the body of the bone, passes downward
and forward horizontally, until it finds an exit at the mental foramen.
This canal lies immediately below the alveolar sockets, and from it are
given off smaller canals which open into the tooth-sockets through
minute foramina. Near the mental foramen the canal divides into a
number of smaller ones, which pass forward through the substance of the
bone to the sockets of the cuspid and incisor teeth.

The Superior Border of the Ramus. — This border is crescent-shaped,
and is otherwise known as the sigmoid notch. Arising from its anterior
portion is a flattened, cone-shaped process — the coronoid process. On
its posterior portion is a rounded or oblong eminence — the condyloid
process. The concave or crescent-shaped margin of this border is thin
and smooth in front, becoming wider and heavier as it approaches the
condyle.

The Coronoid Process. — The anterior margin of this process, being
a continuation of the external oblique line, is heavier at the base than at
the apex. The outer surface is smooth, and affords attachment to the
masseter, and a few fibers of the temporal muscle. The internal surface
is marked by a vertical ridge, which passes downward, increasing in size,
and finally joining the internal oblique line at a point posterior to the
third molar. The surface anterior to this ridge is grooved, and gives
attachment to a part of the temporal muscle above, and the buccinator
muscle below. The surface posterior to this ridge affords attachment
for the greater part of the temporal muscle. The posterior border of
this surface is thin, and forms the anterior margin of the sigmoid notch.

The Condyloid Process. — This may be described as the expanded
extremity of the posterior border of the ramus, and is quite variable in
form (see Occlusion of the Teeth). It is divided into -a superior or
articular portion, and an inferior portion, or neck.

The articular portion of the condyle is more or less oblong, and i>
convex above, fitting into the glenoid fossa of the temporal bone, and

INFERIOR MAXILLARY BONE. 57

forming, with the interarticular cartilage which lies between the two sur-
faces, the temporomaxillary articulation.

The neck is that constricted portion immediately below the articular
surface. It is flattened in front and presents a pit — the pterygoid fossa —
to which a portion of the pterygoid muscle is attached. Immediately
below the point of junction between the neck and the articular surface
externally is the condyloid tubercle, to which is attached the external
lateral ligament.

The Inferior Border of the Ramus. — This border is thick, rounded, and
continuous with the lower border of the body of the bone. At the point
of junction between this and the posterior border is the angle of the jaw.
The angle has a slight outward inclination, and is roughened for the
attachment of a part of the superficial portion of the masseter muscle.

The anterior border has been described in connection with the coro-
noid process.

The posterior border is smooth and rounded on its upper half, the
lower half being roughened for the attachment of the stylomaxillary
ligament.

Attachment of Muscles. — The following muscles are attached
to the mandible:

Buccinator, Superior constrictor of pharynx,

Depressor labii inferioris, Masseter,

Depressor anguli inferioris, Orbicularis oris,

Levator menti, Internal and external pterygoid,

Geniohyoglossus, Geniohyoid,

Platysma myoides, Mylohyoid,

Digastric, Temporal.

Development.* — On account of its early functional activity, the
mandible is among the first bones to ossify. Development takes place
from six centers for each lateral half, the nuclei being deposited as early
as the sixth or eighth fetal week, and after their establishment the develop-
mental process takes place very rapidly. The six centers of ossification
are principally named according to their position. The early preparation
for the development of the bone is found in the appearance of what is
known as the mandibular plates, which are thrown out from the sides
of the cranial base, and finally unite at the median line. Not long after
this period a cartilaginous band — Meckel's cartilage — is developed in the
substance of the mandibular plates, and it is about this cartilaginous

* See " Development of the Teeth."

ANATOMY.

framework that ossification first takes place. The various centers are
distributed along the line of Meckel's cartilage, and are named as follows:
Mental, dentinary, coronoid, condyloid, angular, and splenic. The
mental center provides for the development of that portion of the bone
between the median line and the mental foramen. The dentinary center

Childhood

Adult

Senile

Fig. 22. — Chart Showing the Evolution and Degeneracy of the Mandible.

forms the lower border and outer plate, and provides for the establishment
of the crypts inclosing the developing tooth-follicles.

The coronoid and condyloid centers are both instrumental in con-
structing these processes, and the angular center provides for the angle of
the bone. The splenic center is somewhat later in making its appearance,
and from it the inner plate of the mandible is formed, the line of union

THE HYOID BONE. 59

between it and the dentinary center being indicated by the mylohyoid
groove. While, as above stated, most of the centers of development are
along or near the line of Meckel's cartilage, the condyloid and coronoid
processes are developed from other cartilage. Soon after birth the two
lateral halves of the mandible begin to coalesce at the median line, this
union taking place from below upward; and by the eighth or tenth month
union is complete and the individual bone is established. The inferior
maxilla is subject to a continuous change in form, not only in regard to
its general contour, but also accommodating itself to the movements and
growth of the teeth, the former taking place at or about the angle, while
the latter occurs in the alveolar portion of the bone.

Figure 22 represents the changes which take place in the angle of
the mandible from youth to old age. It will be observed that the angle
formed in the adult bone, with the teeth in position, is almost a right
angle; and that in youth, with the deciduous teeth in the alveoli, the angle
is much more obtuse, which condition is again approached in old age.

THE HYOID BONE.

This is a U-shaped bone placed in the upper part of the neck at the
median line near the base of the tongue. It has no bony connection
with other bones; it is classed as a floating bone. It is made up of a body
and four processes.

The body, or central portion, is quadrilateral in outline, somewhat ob-
long from side to side, with its anterior aspect convex, and presents a
longitudinal ridge which divides it into a superior and an inferior portion.
It is also usually divided at the median line by a slight vertical ridge.
At the point of junction between the longitudinal and vertical ridges a
slight tubercle is formed. The anterior surface is given up to the attach-
ment of muscles. The posterior surface of the body of the bone is con-
cave and smooth, and is directed backward and downward. The superior
border gives attachment to the thyrohyoid membrane, while the inferior
border, which is somewhat thicker, gives attachment to the sternohyoid
and thyrohyoid muscles.

The processes known as the greater and lesser cornua are four in num-
ber, one of each kind on either side.

The greater cornua project backward and upward, and their lower
borders and anterior surfaces are occupied with muscles. The thyro-
hyoid ligament is attached to the posterior terminal corner.

The lesser cornua are short conical pieces of bone, and project upward

60 ANATOMY.

and backward from the extremities of the body of the bone. They
give attachment to the stylohyoid ligaments.

Development. — Ossification takes place from five centers, one for
the body of the bone and one for each cornua.

Attachment of Muscles. —

Geniohyoglossus, Sternohyoid,

Geniohyoid, Lingualis,

Thyrohyoid, Omohyoid,

Mylohyoid, Digastric,

Hyoglossus, Middle constrictor.

The thyrohyoid ligament, as well as the stylohyoid, and the thyrohyoid
membrane are also attached to this bone.

CHAPTER III.

The Temporomandibular Articulation. — The Muscles of

Mastication.

TEMPOROMANDIBULAR ARTICULATION.

Although external to the cavity of the mouth, this articulation is so
closely associated with the masticatory function that it seems important
that a brief description of its construction and action should be given.
It receives its name from the two bones which enter into its formation —
the temporal and the mandible, or inferior maxillary.

Condyle

Ramus of
Mandible

Glenoid
Fossa

Coronoid
Process

Fig.

-Temporomandibular Articulation.

This joint is the seat of motion in the mandible, and entering into its
construction are bones, ligaments, cartilage, and synovial membrane,
these being the tissues essential to all diarthrodial or movable articulations.
The various movable joints of the body are classified according to the

62 ANATOMY.

nature of the movement, and correspond to the mechanical actions known
as hinge joint, ball-and-socket joint, gliding joint, pulley joint, etc. The
temporomandibular joint is of the diarthrodial class, and the movements
which it possesses are a combination of the gliding movement (arthrodia)
and of the hinge movement (ginglymus). The osseous parts entering
into the formation of the joint are the anterior portion of the glenoid
fossa of the temporal bone and the condyloid process of the mandible
(Fig. 23).

The glenoid fossa may be described as an oblong cavity, with its
base directed upward, being bounded anteriorly by a heavy bone ridge
(the anterior root of the zygoma), posteriorly by an irregular, flattened
portion of the bone (the tympanic plate of the petrous portion), internally
by a union of the anterior and posterior boundaries, and externally by the
middle root of the zygoma. The floor of the fossa is traversed by a well-
marked fissure — the glenoid fissure (fissure of Glaserius) — which divides
the fossa into two portions, an anterior and a posterior. The anterior
half is deeper and more concave than the posterior, and is the articulating
portion, being occupied by the condyle, while the posterior half gives
lodgment to the parotid gland.

The condyloid process of the mandible having been described
with that bone, in this connection reference will be made to the variety
of forms which it presents, and the influence which it exerts over the
nature of the tooth occlusion. This process, when narrow and oblong
(Fig. 23), closely resembles the ginglymus, or hinge joint, and will be
accompanied by teeth presenting deep, penetrating cusps, forming a
positive and well-locked occlusion (Fig. 24, B), with little or no lateral
motion. If the condyle presents the appearance shown in figure 25,
which resembles the enarthrodia, or ball-and-socket joint (although it
cannot be considered as such), the teeth associated with such a formation
will be provided with short, rounding cusps, and the occlusion will be
loose and wandering (Fig. 24, A). This difference in the form of the con-
dyle will be accompanied by a corresponding variation in the concavity
of the glenoid fossa. Not only does the osseous structure in the joint
partake of individual characteristics, but likewise the muscles and liga-
ments; their functions being to operate the articulation, they are developed
in accordance with the action required of them, which action is, in a
measure, dependent upon the conditions existing in the mouth.

Both the condyle and the glenoid fossa are covered with articular car-
tilage. In the latter this membrane extends over its anterior border, to
facilitate the play of the joint. The condyle is held in position in the fossa

TEMPOROMANDIBULAR ARTICULATION.

by three ligaments — the capsular, the sphenomaxillary, and stylomaxillary.
The capsular ligament is divided into four portions — anterior and posterior,
external and internal. The anterior portion consists of a few fibers

Fig. 25.

connected with the anterior margin of the fibrocartilage, attached below
to the anterior margin of the condyle and above to the front of the glenoid
ridge. The posterior portion is attached above just in front of the glenoid

6 4

ANATOMY.

fissure, and is inserted into the posterior margin of the ramus of the
maxilla just below the neck of the condyle. The external portion,
otherwise known as the external ligament, is the strongest portion of the
capsular ligament. It has a broad attachment above to the zygoma,
from which point it passes downward and backward, and is inserted into
the outer side of the neck of the condyle. The internal portion, or short

Styloid Process Capsular Ligament

Internal Lateral Ligament
/

Stylohyoid Ligament

Stylomaxillary Ligament
Fig. 26. — Temporomaxillary Articulation — Internal View. (Denver.)

internal lateral ligament, is composed of well defined libers, having a broad
attachment above to the inner edge of the glenoid fossa and to the alar
spine of the sphenoid bone; below it is inserted into the inner side of the
neck of the condyle.

The sphenomaxillary, or long internal lateral ligament, is a thin,
loose band, situated some distance from the joint proper, and, as its
name implies, has its attachment above to the alar spine of the sphenoid

TEMPOROMANDIBULAR ARTICULATION.

bone, and also to that portion of the temporal bone contiguous to it. It
passes downward and forward, and is inserted into the mandibular spine
of the maxilla.

The stylomaxillary ligament extends, from the styloid process of
the temporal bone, downward and forward, to be inserted into the poste-

External Lateral Ligament

Capsular Ligament

Styloid Process
Stylohyoid Ligament
Stylomaxillary Ligament

Fig. 27. — Temporomaxillary Articulation — External View. (Denver.)

rior border of the ramus of the inferior maxilla, at a point between the
masseter and internal pterygoid muscles.

The inter -articular fibro cartilage is an oval sheet placed between the
two articulating surfaces. It is thinnest at the center and becomes thicker
as the margins of the fossa are approached, at which point it is connected
with the fibers of the capsular ligament. Being placed immediately be-

ANATOMY.

tween the two articular surfaces it divides the joint into two separate
synovial cavities. Each of these synovial cavities is occupied by a syno-
vial membrane, that occupying the upper compartment being the largest,
and passes from the margins of the glenoid fossa above to the upper
surface of the interarticular cartilage below. The membrane which
occupies the lower cavity is smaller, and passes from the under surface
of the interarticular cartilage above to the margins of the condyle below.
The blood-supply to this articulation is derived from the temporal, middle
meningeal, and ascending pharyngeal arteries.

Tin- nerves are derived from the masseteric and auriculotemporal.

Fig. 28.— Showing Variation in the Shape of the Condyles.

The movements of this articulation present as great a range as any
other joint in the human body. While the chief movement is of the
ginglvmoid or hinge character, brought into play in simple depression
and elevation of the mandible, it also has the power of extension and
retraction, may be rotated from side to side, together with all the motions
intermediate between these. When the mandible is depressed, the con-
dyle moves on the fibrocartilage, and at the same time glides forward and
slightly downward until it rests on the anterior border of the glenoid
fossa; — this movement does not extend sufficiently to allow the condyle
to rest upon the extreme summit of the border, except in cases of excessive
movement, as in yawning, when the condyle may glide over the summit

TEMPOROMANDIBULAR ARTICULATION.

6 7

and the joint become disarticulated. When the mandible is elevated, the
condyle slides backward and upward, and at the same time the fibro-
cartilage, which has extended with it, also retracts until the condyle is
settled in the fossa. The movement of extension and retraction is by a
horizontal gliding action, by which the mandible is thrust forward and

Temporal Muscle

Buccinator Muscles
Masseteric Nerve
Masseteric Artery

Facial Artery
Facial Vein

Superficial Temporal

Artery
Facial Nerve
Masseter Muscle

Platysma Myoides
Muscle

Fig. 29. — Temporal Muscle. (Deaver.)

drawn back again. In this movement, as well as in the one previously
described, both condyles are similarly and simultaneously engaged. The
lateral or triturating movement is made in an oblique direction. This
consists in a rotation of the condyles within the fossae, the cartilage gliding
obliquely forward and outward on one side, and backward and inward
on the other, this action taking place alternately. This movement is

68 ANATOMY.

more or less developed in accordance with the nature of the occlusion,
being favored by those teeth possessing but little cusp formation, with a
consequent loose and wandering occlusion; while in that type of tooth
associated with a long overbite and deep penetrating cusps, forming a
firm and well-locked occlusion, this movement will be but little developed.
If this movement be employed to throw the symphysis to one side and
back again, and not from side to side, the condyle of that side rotates in
the glenoid fossa, while the condyle of the opposite side is drawn forward
and inward.

The Muscles of Mastication.

Occupying the back part of the side of the face, and forming an in-
dependent group, are four muscles, usually classed as the muscles of
mastication. While this is true to a great degree, they are not the only
muscles brought into action during this process. They are the masseter,
temporal, internal pterygoid, and external pterygoid. The masseter,
temporal, and internal pterygoid lift or close the lower jaw, the principal
function of the external pterygoid being to extend the lower jaw so that
the lower teeth pass beyond the upper. The muscles which open the
jaws, such as happens when the head is thrown backward, are the muscles
of the neck.

The masseter muscle is made up of a strong quadrate sheet, con-
sisting of two distinct layers extending from the zygomatic arch to the
mandible. The layers of which the muscle is composed differ somewhat
in size as well as in the directions which they take.

Origin. — The superficial layer, much the stronger and larger of the
two, arises from the lower border of the malar bone, and from the anterior
two-thirds of the zygomatic arch. The deep layer arises from the posterior
third of the lower border and from nearly all the internal surface of the
zygomatic arch.

Insertion. — After passing downward and backward it is inserted into
the outer surface of the ramus of the mandible. The deep layer, passing
downward and slightly forward, mingles with some of the fibers of the
superficial portion, and is finally inserted into the upper half of the ramus
of the mandible.

Action. — To draw slightly forward, and by its superficial layer to
close the jaw. " In closing the jaw it acts with less mechanical disad-
vantage than is usual with muscles. When the pressure to be overcome
is exerted upon the back teeth, the arm of the lever upon which the power
acts is almost as long as that which intervenes between these teeth and the

TEMPOROMANDIBULAR ARTICULATION.

6 9

fulcrum. This fulcrum is not at the temporomaxillary joint, but at a
point below the neck of the mandible, corresponding very nearly to the
lower attachment of the internal lateral ligament. Moreover, the result-
ant force of the muscle acting, as it does, upward and forward, is perpen-
dicular to the lever, which may roughly be described as a bar extending
downward and forward from the neck of the mandible to the point of the
chin." (Morris.)

External pterygoid

Int3rnal pterygoid

Interartieular flbro-cartilage

j ■

Fig. }o. — The Pterygoid Muscles. (Morris.)

Relations. — It is covered superficially by the skin and fascia of the
platysma myoides, by the risorius and the masseter fascia, by the parotid
gland and ducts, facial veins, and portions of the facial nerve. Deeply, the
muscle lies in contact with the ramus of the jaw and the buccinator muscle,
being separated from the latter by a layer of fat. A small portion of the
temporal muscle also comes in relation to the deeper lying portion.

The temporal muscle is covered by a strong membrane, the temporal
fascia, which arises from the temporal ridge, and is inserted into both the
inner and outer portions of the upper border of the zygomatic arch. Near
this point it divides into two distinct layers, one passing to the inner, the
other to the outer margins of the zygoma. Below the fascia is continuous

70 ANATOMY.

with the masseteric fascia. Passing downward from this to the inferior
borders of the ramus of the jaw, it envelops the masseter muscle. The
muscle itself is radiating and fan-shaped in form, located in the temporal
fossa, from which point it descends to the coronoid process of the
mandible.

Origin. — From nearly the entire surface of the temporal fossa, from
the temporal ridge, from the entire surface of the temporal fascia,
down to its lower attachment to the zygomatic process.

Insertion. — -Into the coronoid process of the mandible.

Action. — To close the lower jaw, some of its fibers drawing the jaw
backward after the other muscles have protruded it.

Relations. — Its superficial portion is covered by the temporal fascia
which separates it from some of the auricular muscles; branches of the
facial nerve, the auriculotemporal nerve, and a portion of the epicranial
aponeurosis. The temporal fossa and the external pterygoid muscles
are in relation with it deeply.

The internal pterygoid muscle is a thick, quadrilateral, sheet-
like muscle, and receives its name from its origin and relative position.

Origin. — From the inner surface of the external pterygoid plate; the
tuberosity of the palate bone and a small portion of the maxilla.

Insertion. — Into the internal surface of the ramus of the mandible at
its lower and posterior borders and extending as high as the mandibular
foramen and mylohyoid.

Action. — To close the jaw and at the same time draw it backward and
throw it toward the opposite side. "The same remarks which were
made with respect to the very small loss of mechanical advantage in the
masseter muscle apply to this muscle. When closed it will draw the
jaw forward; and also it will help the external pterygoid in drawing the
ramus of its own side toward the middle line." (Morris.)

Relations. — Superficially, the internal maxillary vessels, the external
pterygoid muscle, the internal lateral ligament, the inferior dental and
lingual nerves. Deeply, the submaxillary glands; the tensor palati and
superior constrictor of the pharynx, as well as the stylohyoid and posterior
border of the digastric muscles.

The external pterygoid muscle is composed of two triangular sheets,
one passing in a horizontal and the other in a vertical direction. It re-
ceives its name from its attachment to the pterygoid process of the
sphenoid bone as well as its relation to its companion muscle, the internal
pterygoid.

Origin. — It is composed of two distinct heads, an upper and a lower.

TEMPOROMANDIBULAR ARTICULATION. 7 1

The upper head arises from the greater wing of the sphenoid bone, from
the internal pterygoid ridge, and external to the foramen ovale and fora-
men spinosum. The lower head arises from the outer surface of the ex-
ternal pterygoid plate.

Insertion. — The upper head is inserted into the inter-articular fibro-
cartilage, into the capsule of the joint as well as the neck of the condyle.
The lower head is inserted into the neck of the condyle.

Action. — To draw the condyle and inter-articular fibrocartilage for-
ward and inward. "The combination of these two movements produces
the oblique movement of the lower molar teeth of one side, forward and in-
ward with respect to the upper molars which are their opponents. It
should be observed also that this inward movement of one side is the
agent by which the ramus of the opposite side is moved outward. To
assist in opening the mouth by depression of the lower jaw. As the
transverse axis of this movement passes through the mandible at two
points situated below the necks of the rami, it follows that a forward
movement of the condyles and necks will assist in the backward move-
ment of the angles and body which accompanies the depression of the
mandible." (Morris).

Relations. — Superficially, some of the fibers of the internal pterygoid,
the temporal, and part of the masseter muscle. Deeply, the internal
pterygoid muscle, the middle meningeal and inferior dental vessels, in-
ternal maxillary vessels, and masseteric and posterior deep temporal
nerves passing behind or through the attachment to the upper head; the
inferior dental and lingual gustatory nerves beneath the lower head.

CHAPTER IV.

A General Description of the Teeth. — The Permanent Teeth:
Classification, Surfaces, Etc. — The Roots of the Teeth.
The Dental Arch.

A GENERAL DESCRIPTION OF THE TEETH.

Apex

A Tooth (Fig. 31). — One of the thirty-two specialized organs for the
seizure and mastication of food, placed at the entrance to the alimentary
canal (the mouth). The typical form of a tooth is a modified cone or

combination of cones, and is composed of
two fundamental parts — the crown and the
root or roots. The crown is that part which
projects beyond the gum and is visible in
the mouth; while the root is that part which
is implanted in the bone and covered by
the gum. Intervening between these two
extremities, and usually occupying a por-
tion of each, is a third division — the neck.

Completely covering the crown of a
tooth is a hard, vitreous-like substance,
enamel;* the root is covered by a hard,
bone-like substance, cementum;* while the
interior or body of the organ is composed
of a hard substance closely resembling bone,
the dentin.* The neck of a tooth, which
serves to unite the crown to the root, and
which is usually formed at the expense of
each, is covered partly by enamel and partly
by cementum. This brief description in the singular number can best be
continued in the plural. Teeth are classified according to their form, which
is always in accordance with their function, into simple and complex. In
the simple class the single mod'fied cone is the predominating form, the free
extremity of the crown serving as the base of the cone, while the apex i-
formed by the free end of the root. Included in this same classification

Fig. ,}i

*See Tissues of the Teeth, Pari II.

A GENERAL DESCRIPTION OF THE TEETH. 73

are those teeth which are made up of a double cone, or a simple cone and
an inverted cone attached to each other at a common base (Fig. 31).
The purposes for which such teeth are adapted are those of grasping,
incising, and tearing, and they are usually so arranged that the free ex-
tremities of their crowns interlock or overhang the opposing teeth in the
opposite jaw.

In the complex class (Fig. 32) the external form of the tooth is pro-
duced by a combination of cones, some of which are simple, others inverted,
but all uniting at a common base — the neck
of the tooth. In this class the simple cones
form the roots of the tooth, while the crowns
are made up of a number of smaller cones,
much modified. Such teeth are adapted to
crushing and grinding, and are less inclined
to interlock during active service.

The teeth are divided into two grand
divisions — those of infancy and childhood,
called deciduous or temporary teeth, and those
of the adult period, known as permanent teeth.
The latter class being most important, will
first receive consideration.

The Permanent Teeth (Fig. 33).— The
permanent teeth, thirty-two in number, are divided into those of the
superior portion of the mouth, upper, and those of the inferior portion,
lower. In number they are equally divided, each jaw giving support to
sixteen. They are firmly imbedded in the alveolar sockets of three of
the bones of the mouth, the upper sixteen being attached to the two
superior maxillary or upper jaw-bones, and the lower sixteen to the
mandible, inferior maxillary, or lower jaw-bone. As above referred to.
the attachment of the teeth to the bones is by implantation in sockets,
the alveoli (see description, " Bones of the Mouth"). In this attachment
there is a special development of bone, closely modeled to the roots of
the teeth, and which is subservient to the ever-varying changes which
take place during the development of the organs. The joint thus
formed between the roots of the teeth and the alveoli is of the immovable
or synarthrodial class, and is styled gomphosis. Intervening between the
roots of the teeth and the walls of the alveoli is a delicate membrane
the alveolodental membrane.

Before continuing the description of the teeth, a further classification,
which refers alike to the upper and lower organs, must be presented.

Fig. 32.

ANATOMY.

This classification is derived from the function and form of the teeth.
Figure 33 shows the thirty-two teeth removed from the jaws and placed
side by side in two straight lines. In the center is a perpendicular line,
which corresponds to the median line or center of the mouth, the teeth
at either extremity being those which occupy the back part of the mouth.
Without confining the description to either the upper or lower teeth, it will
be observed that the first two teeth upon either side "of the median are
similarly formed, and all four are called incisors (incidere, to cut); the
two larger incisors, being nearest the median line or center, are called

Lingual Surfaces

Labial and Buccal Surfaces

Uppe

Lower

Lingual Surfaces

Fig. 35

Labial and Buccal Surfaces

-The Permanent Teeth.

central incisors; while the two smaller being placed at the side of the cen-
trals, are known as lateral incisors {lateralis, the side). The third tooth
from the median line upon either side is the cuspid (cuspis, a point), so
named from possessing a single cusp or point. Passing to the right or left
on the chart, or backward in the mouth, the fourth and fifth teeth from
the median line are the bicuspids (bi, two; cuspis, a point), having two
points or cusps. The bicuspid nearest the median line is the first
bicuspid; that most distant from the median line is the second bicuspid.
The sixth, seventh, and eighth teeth from the median line upon either
side are those of another class, the molars (mola, a mill-stone), being
named according to their function, that of crushing or grinding the
food. Proceeding from before backward, the molars are denominated

A GENERAL DESCRIPTION OF THE TEETH. 75

first molar, second molar, and third molar. To sum up, the names and
number of the permanent teeth may be given by the dental formula, as
follows :

Incisors, \ Bicuspids, \

Cuspids, i Molars, §

The Surfaces of the Teeth. — The crown of each tooth presents
five surfaces, which are variously named, in accordance with the duty
which they are called upon to perform or suggestive of their location.
The outer surface of the incisors and cuspids, or that contiguous to the
lips (labia), is called the labial surface; the corresponding surface of the
bicuspids and molars, or that contiguous to the cheeks (buccae), is the
buccal surface. That surface of both upper and lower teeth which faces
the palate or tongue is characterized as the lingual surface.

The proximate surfaces of the teeth are named with regard to their
relation to the median line, those surfaces nearest to this point being
called mesial, those most distant, distal. In addition to these four sur-
faces, which represent what might be termed the sides of the teeth, a fifth
surface is present, that which occludes with the teeth of the opposite jaw,
and is called the occlusal surface.

In the incisors and cuspids this surface is formed by the converging
of the labial and lingual surfaces, forming an edge to the free extremity of
the crown, named, from its action in mastication, the incisive or cutting-
edge. In the bicuspids and molars the various sides of the crowns
remain nearly parallel to each other throughout their extent, thus provid-
ing a surface nearly equal to, or greater than, any of the others, and one
well adapted to the purposes for which it is intended — that of grinding or
crushing the food.

The Roots of the Teeth. — The upper incisors and cuspids are each
provided with one root; the upper first bicuspid may have one or two
roots, most frequently the latter; while in the second bicuspid a single root
is usually present. The upper first and second molars are each supported
in the jaw by three roots, and while in the upper third molar three roots
are most common, the number is quite variable, ranging from a single
cone-shaped root to three, four, five, or even six smaller branches given off
from a common base.

In the lower incisors, cuspids, and bicuspids, a single root is most
frequently met with, although the latter class, in rare instances, may be
provided with two. The lower first and second molars are each provided
with two roots, but in the third molar, like its upper fellow, the number

7 6

ANATOMY.

may be diminished or increased. In the upper molars, two of the three
roots are placed above the buccal half of the crown, and are called buccal
roots; the remaining root is placed above the lingual half of the crown,
and is designated as the lingual root. In the lower molars, one of the
two roots is placed below the anterior or mesial half of the crown, and is
named the mesial root, and the other below the posterior or distal half,
and is known as the distal root. In those teeth with a complicated root

formation, it would seem to be a
question whether they are pos-
sessed of a single root, with two
or more branches, or separate
and distinct roots throughout.
To determine this, some account
must be taken of the point at
which the bifurcation or trifurca-
tion takes place. If this separa-
tion be in close proximity to the
crown, the tooth should be con-
sidered as having more than one
root (Fig. 34, A) ; but, on the
other hand, if the point of sepa-
ration be some distance from the
crown, with a solid mass of root substance intervening, the tooth may
be said to possess a single root, with two or more branches (Fig. 34, B).
In the latter instance, that part of the tooth between the point of
reparation and the crown is called the root or root base; while the
prolongations beyond the point of separation are known as the branches
of the root.

The roots of the teeth are not only variable in number, but are also
subject to much variety in form. In the anterior teeth (the incisors and
cuspids) the roots are inclined to the form of the simple cone, which form,
however, is frecpjently more or less broken by a slight curvature near their
extremities, or by a slight compression of their lateral walls. In the poste-
rior teeth (the bicuspids and molars) the roots, root bases, or root branches
are all inclined to the conical form, but do not approach so nearly the
perfect cone as those of the anterior teeth. These roots are also more or
less crooked and flattened laterally. The free extremity of the roots
of the teeth, forming as they do the apex of these cone-like prolongations
of the crowns, are known as the apices or apical extremities.

The extent of the enamel covering to the crowns of the teeth is

Fig. 34.

A GENERAL DESCRIPTION OF THE TEETH.

marked by a well-defined line, which completely encircles the neck of the
tooth, the cervical line.

The Dental Arch (Fig. 35). — The teeth are arranged in the jaws
in the form of two parabolic curves, the superior arch describing the
segment of a larger circle than the inferior, as a result of which the upper
teeth slightly overhang the lower. Figure 35 represents the sixteen
upper teeth in position in the bone, presenting their occlusal surfaces, a
part of their lingual surfaces also being visible. Viewed in this direction,
the gradual change in the crowns of the teeth from the simple incisors to

Incisors

Cuspid

3d Molar

Fig. 35. — The Dental Arch.

the complex molars may be observed. An examination of the central
incisors will show how perfectly they are adapted to the process of cutting
or incising the food, the cutting-edge being sharp and the lingual surface
comparatively smooth and unbroken. In the lateral incisors the cutting
feature predominates, but the lingual surface is broken near the neck of
the tooth by a slight depression, surmounted by a more or less pronounced
fold of enamel, in many instances resembling a small cusp. The crown
of the cuspid tooth furnishes the intermediate form between the simple
and the complex. This tooth, instead of being provided with a straight
cutting-edge, is surmounted at the center of its occlusal surface with a
well-defined point or cusp, descending from the summit of which are two

78 ANATOMY.

cutting-edges, one passing to the mesial and one to the distal. The
lingual surface of this tooth presents a marked contrast to the correspond-
ing surface of the incisors, being broad and full, and frequently provided
with a prominent ridge of enamel in the region of the neck, showing a
rapid approach to the complex form. In the bicuspids the buccal half
of the crown is quite similar to the crown of the cuspid, but in the lingual
half a complete revolution has taken place. The enamel fold — but slightly
apparent in the incisors, and somewhat increased in the cuspids — has
now become a fully developed cusp, resulting in the production of an
occlusal surface adapted to crushing or grinding, instead of incising or
tearing. In the molars, the increase in the size of the tooth-crown is ac-
companied with an occlusal surface much more complex than any of
the teeth previously described, and one well adapted to its function-
that of crushing and grinding the food.

Arrangements of the Teeth in the Dental Arch (Fig. 35). — Beginning
with the upper teeth, the central incisors are found occupying the center
of the arch, and are, therefore, slightly in advance of the laterals. These
teeth are so implanted in the alveoli that their crowns are not perpendic-
ular, the cutting edge being slightly more prominent than the neck of
the tooth. The roots are also somewhat inclined from the median line,
and as a result the crowns have a slight mesial inclination, the mesial sur-
faces approximating each other at or near the cutting-edge, with a slight
space intervening at the necks. In certain typal forms — the bilious, for
example — when the front of the arch is flat, the labial surfaces of these
teeth form nearly a direct line from side to s.'de; while in those types in which
the arch is well rounded anteriorly, notably in the sanguine temperament,
the labial surfaces of these two teeth form a small segment of the arch,
so that the mesial extremity of the cutting-edge of each tooth-crown is
somewhat in advance of the distal. The lateral incisors are similarly
implanted in the alveoli, causing their cutting-edges to project. The
roots of the lateral incisors usually have a stronger distal inclination than
those of the centrals, and the crowns show a more marked mesial inclina-
tion. The mesial surfaces approximate the distal surfaces of the central •
incisors at or near the cutting-edge. When the front of the arch is flattened,
these teeth are but little less prominent than the central incisors, but when
the arch is well rounded they continue the segment begun by the centrals,
and are necessarily less prominent. While the occlusal surfaces of the
teeth are usually considered as forming a perfect plane (see Occlusion of
the Teeth), the lateral incisors are generally a trifle shorter than the
centrals. The cuspids may be considered as occupying the corners or

A GENERAL DESCRIPTION OF THE TEETH. 79

turning-points of the arch. They are more prominently placed than the
adjoining teeth, this feature being increased by the bulging or general
convexity of their labial surfaces. The extremity of the occlusal surface
of the cuspids — i. e., the point of the cusp — is a trifle below the cutting-
edge of the laterals and about on a line with that of the centrals. While
the apical extremities of the roots of the cuspid teeth are directed away
from the median line, the crowns assume 'almost a perpendicular, this
condition resulting from a bend in the tooth at the neck. Although the
perpendicular position is most commonly assumed by the crown, it is not
unusual to find either a mesial or distal inclination present. Reference
has been made to the cuspid teeth occupying a position which might be
termed the turning-points or corners of the arch, and in most instances
it may be thus considered; but in certain typal forms — the sanguine, for
example — the tooth-line is unbroken and passes over the cutting-edges
of the incisors, the summit of the cusps of the cuspids, and is continued
backward over the buccal cusps of the posterior teeth. The bicuspids
are placed nearly perpendicular in the arch, but occasionally deviate
from this by a slight mesial or buccal inclination. The length usually
corresponds to that of the central incisors, and their buccal surfaces are
slightly less prominent than the corresponding surfaces of the cuspid
teeth. The increase in the buccolingual diameter of the crowns of
the bicuspids over that of the incisors and cuspids results in breaking
the lingual line of the occlusal surfaces. In the bilious and kindred
types, the tooth-line is carried directly backward from the cuspid to
the first molar, making the buccal face of the bicuspids equally promi-
nent, but when the arch is well rounded the second bicuspid is
slightly more prominent than the first. The first and second molars
usually assume a perpendicular position, but are occasionally inclined to
the distal and buccal. The relative prominence of the buccal as well as
the lingual surfaces of these teeth is also controlled by the form of the arch.
The occlusal surfaces are about on a level with those of the bicuspids
and central incisors, but generally the lack of development in the distal
half of the crown of the second molar results in the production of a slight
upward curve to the tooth-line level at this point (see Occlusion of the
Teeth). On account of the limited accommodations afforded it, the posi-
tion of the upper third molar is quite variable. It may be either to the
buccal or to the lingual of the tooth-line, and is usually strongly inclined
to the distal. In those cases in which there is a decided dip to the arch
(see Occlusion of the Teeth), this tooth is relatively shorter than those an-

80 ANATOMY.

terior to it, but when the tooth-line level is a perfect plane, the length of
this tooth corresponds to the other molars and bicuspids.

Fig. 36. — The Tooth-line in the Lower Jaw.

The lower incisors are placed more nearly in a perpendicular position
than the upper, and a reverse condition exists, in the lateral incisors

Fig. 37. — The Tooth-line in the Lower Jaw.

being a trifle larger than the centrals. The lower cuspids are probably
more constant in their position than any class of teeth in the mouth, in

A GENERAL DESCRIPTION OF THE TEETH.

nearly all instances assuming a direct perpendicular. Like the upper cus-
pids, they may be said to establish the corners or turning-points of the
inferior arch, and are somewhat more prominent in the tooth-line than

Fig. 38. — Sanguine.

neighboring teeth. All of the six anterior lower teeth may be slightly
inclined to the mesial. The lower bicuspids and molars, instead of
having the buccal inclination possessed by the corresponding upper teeth,

Fig. 39. — Bilious.

are inclined to the lingual. The first molar seldom deviates either to the
mesial or the distal, the second molar is generally inclined to the mesial,
while the third molar is strongly inclined to the mesial. In the inferior

ANATOMY.

arch the curve formed by the incisors and cuspids is the segment of a
smaller circle than the corresponding curve in the superior arch. This
curve may be continued over the buccal cusps of the bicuspids and molars.

FlG. 40. -Nervous.

or it may be broken at the cuspid tooth and continued backward in a direct
line (Fig. 36). The teeth in the inferior arch are placed directly over the
body of the bone as far back as the second bicuspids, while the molars

» i ~ r .

• «

^^\

' 1

^^^^^^^B

*Jm

T-it

§1

Fig. 41. — Lymphatic.

frequently overhang the body of the bone by an extension of the alveoli
inward (Fig. 37).

The curve described by the dental arch is quite variable, and this

A GENERAL DESCRIPTION OF THE TEETH. 83

variation is generally referred to in connection with the temperament.
Thus, in the sanguine temperament (Fig. 38), the arch is well rounded
anteriorly, the circle being continued backward to the region of the molars,
where the line is broken by slightly inclining to the lingual. In this
arch the distance in a straight line from the center of the second molar,
on one side, to the center of the corresponding tooth on the other, is about
equal to the distance from either of these points to the median line be-
tween the central incisors, forming a right-angle triangle. In the bilious
temperament (Fig. 39) the arch presents a broad front from cuspid to

Fig. 42. — Section of the Superior Maxilla Showing Interproximate Spaces.

cuspid, with but little curve; at these points it turns abruptly backward,
being continued almost in a direct line to its extremity. In this arch
the side of the triangle (represented by the line from molar to molar)
is much reduced in length. In the nervous temperament (Fig. 40)
the arch is Gothic in form, the segment formed by the anterior teeth
being that of a much smaller circle than either of the types previously
referred to. The distance from molar to molar is much less than the
distance from molar to median line. In the lymphatic temperament
(Fig. 41) the arch is well rounded and broad, the segment being that
of a much larger circle than any of the above, the side of the triangle
formed by the line from molar to molar being of the greatest length.

Interproximate Spaces. — In the mesiodistal direction the crowns
of the teeth, as a class, are broader at their occlusal surfaces or cutting-
edges than at their necks (Fig. 42). This bell-shaped form of the tooth-
crowns causes their proximate surfaces to touch at a point representing

84 ANATOMY.

their greatest mesiodistal diameter, which is usually near the cutting-edge
or occlusal surface. Between this point of contact and the cervical line
there exists a V-shaped space, called the interproximate space. These
spaces are largest in that class of teeth found in the nervous and bilious
types, where the necks of the teeth are much constricted, and the bell-
shaped crown strongly outlined. In teeth of this class the point of contact
is slight, and the interproximate spaces are only partially occupied by
the gum tissue, leaving a free passage between the point of contact and
the gingival margins. In the sanguine and lymphatic temperaments the
proximate surfaces of the teeth are nearer parallel with one another, thus
making the point of contact cover a greater extent of surface, and reducing
the size of the interproximate spaces.

CHAPTER V.

Occlusion of the Teeth.

As stated elsewhere, the teeth are arranged in the mouth in the form
of two parabolic curves, one of which occupies the upper half and the
other the lower half of the cavity. To properly perform their function it
is necessary for the upper and lower teeth to come into contact, which

Fig. 43. — The Teeth in Occlusion.

they are enabled to do by the movement of the lower jaw, and it is the
relation existing between the two when thus brought together that con-
stitutes the occlusion of the teeth. During mastication the teeth do not
only occlude, and remain stationary at a given point until the lower jaw
is again depressed, but, through the combined movements of the man-
dible, the lower teeth are made to move from side to side, thus grinding
or crushing any substance placed between the occlusal surfaces of the
bicuspids and molars. This gliding antagonism of the teeth is commonly
termed the articulation, and it is important that a distinction be made be-

86 ANATOMY.

tween the terms "occlusion" and "articulation," the former referring to
the relations existing between the upper and lower teeth when brought
together normally and held firmly in that position, while the latter relates
to the various movements of the teeth after being brought together in
occlusion. In the majority of instances the segment described by the
superior arch is somewhat larger than that formed by the inferior, and
the upper teeth project over and are partly outside of those in the inferior
arch. Figure 43 presents a labial and buccal view of the teeth in position
in the alveoli, and also in occlusion. It will be observed that the upper
teeth are not directly antagonistic to those of the same name in the in-
ferior arch. There are two reasons for the presence of this condition:
First, the mesiodistal diameter of the upper central incisors is much
greater than that of the corresponding lower teeth; second, the larger
circle present in the superior arch. This arrangement provides that
each tooth, instead of being antagonized by a single tooth of the opposite
jaw, is met in occlusion by a portion of two teeth. The upper central
incisor is met in occlusion by the entire cutting-edge of the lower central
incisor and the mesial third of the cutting-edge of the lower lateral in-
cisor. The upper lateral incisor is met in occlusion by the distal two-thirds
of the cutting-edge of the lower lateral incisor and by the mesial cut-
ting-edge of the lower cuspid. The upper cuspid is met in occlusion by
the distal cutting-edge of the lower cuspid and by the mesial two-thirds
of the buccal cusp of the lower first bicuspid. The upper first bicuspid
is met in occlusion by the remaining or distal third of the lower first
bicuspid and by the mesial two-thirds of the buccal cusp of the lower
second bicuspid. The upper second bicuspid is met in occlusion
by the remaining or distal third of the buccal cusp of the lower second
bicuspid, and by the mesial incline of the mesiobuccal cusp of the lower
first molar. The upper first molar is met in occlusion by the distal in-
cline of the mesiobuccal cusp of the lower first molar, by the entire distal
cusp of the same tooth, and by the mesial incline of the mesiobuccal cusp
of the lower second molar. The upper second molar is met in occlusion
by the distal incline of the mesiobuccal cusp of the lower second molar,
by the entire distobuccal cusp of the same tooth, and by the mesial incline
of the mesiobuccal cusp of the lower third molar. The upper third
molar is met in occlusion by the distal incline of the mesiobuccal cusp of
the lower third molar and by the entire distobuccal cusp of the same tooth,
thus being the only tooth in the superior arch with but a single opponent.
Likewise each lower tooth is met in occlusion by two in the superior arch,
with the single exception of the central incisor, which occludes with the

OCCLUSION OF THE TEETH.

upper central alone. There are many variations from this so-called
typical occlusion, as above described, and any slight difference one way
or another should not be considered abnormal. In certain types the
segmental form of the superior arch is but little greater than that of the
inferior, and the cutting-edges of the upper incisors occlude almost directly
upon the cutting-edges of. the lower incisors. As a result, all of the upper
teeth are forced to the distal, and the relationship between the upper and
lower organs is much altered. When the upper teeth overhang the lower,
the lingual cusps of the upper bicuspids and molars penetrate the fossae

Fig. 44.

-The Mandible at the Adult Period, showing the Equilateral Triangle
described bv the Dental Arch.

or sulci of the corresponding lower teeth, when in occlusion, and the
buccal cusps of the lower bicuspids and molars rest in the fossae of their
upper opponents.

To assist in the study of the occlusion of the teeth, some reference
must be made to the tooth-line lever, or plane of occlusion. For this
purpose the lines forming the facial angle are of value. These lines are
as follows: A fixed line representing the base of the angle may be drawn
from the center of the glenoid fossa, passing forward through the anterior
nasal spine or base of the nose (a, Fig. 45), the angle being completed by a
perpendicular line resting upon the labial surface of the upper incisors,
passing upward and touching the most prominent part of the forehead (b,
Fig. 45). The tooth-line level is approximately horizontal to this basal
line, but instead of a perfect plane we usually find the superior arch dipping
downward, while the inferior arch will be provided with a corresponding

ANATOMY.

depression. This dip to the arch is greatest in the region of the bicuspids,
and the extent to which it may exist varies with the type of tooth and the
consequent nature of the occlusion.

Fig. 4.5. Lines showing Facia] Angle, Caucasion or White Race

Thus far no reference has been made to what is commonly termed
the overbite, and the cusp forms in the teeth. As these two factor exert

/ .i*

1 M^ A

•

/ lytvor^

/ M^ *

Fig. 46. — Lines showing Facial Angle, Negro or Mixed Races.

a dominating influence over the character of the occlusion, the effects which
they produce will be briefly described. The overbite is so named, from the

OCCLUSION OF THE TEETH.

8 9

fact that the upper teeth project beyond, or overhang and partly cover the
labial and buccal surfaces of the lower teeth. This may be a pronounced
feature in the tooth-occlusion, or it may exist to a very slight degree.
Although the overbite is usually referred to as existing in the incisive
region alone, it is not confined to these teeth, but is also present in the
bicuspids and molars by the buccal cusps of the upper teeth overhanging

Fig.

-Lymphatic.

those of the lower. The extent of the overbite is gradually diminished
from before backward, the central incisors presenting the greatest amount
of overhanging surface, which condition is slightly decreased in the
laterals, and a corresponding reduction is continued until the third
molars are reached, at which point the overbite is scarcely observed.
Where the overbite is extensive, as shown in figure 43, the upper incisors

Fig. 48. — Sanguine.

overhanging and hiding from view about one-third of the labial surfaces
of the lower incisors, the cusps of the bicuspids and molars will be corre-
spondingly long and penetrating, the buccal cusps of the upper teeth ex-
tending well down over the buccal cusps of the corresponding lower
teeth. In an occlusion of this class, which is usually found in the ner-
vous and bilious types, the dip to the arch will become a prominent

9°

ANATOMY.

feature, the occlusion will be firm and well locked, and the lateral articular
movements will be slight during mastication. In the lymphatic and san-
guine temperaments the occlusion is loose and wandering, greater freedom
of movement being permitted by the short overbite and the corresponding
lack of cusp-formation. Figure 47 represents such an occlusion; the

Fig. 49. Bilious.

superior arch is but little greater in its segmental outline than the inferior.
The cutting-edges of the upper incisors are somewhat more prominent
than those of the lower, but the former do not overlap the labial surfaces
of the latter. In an occlusion of this character the dip to the arch is not
so pronounced, and the articular movements are much more extensive.

CHAPTER VI.

The Blood- and Nerve-Supply to the Teeth.

THE BLOOD-SUPPLY TO THE TEETH.

Briefly stated, the course of the blood from the heart to the teeth is
as follows: From the heart to the aorta, to the common carotid artery,
to the external carotid artery, to the internal maxillary artery, from the
various branches of which the teeth are supplied.

The Internal Maxillary Artery (Fig. 51). — This artery, otherwise
known as the deep facial, is the larger of the two terminal branches of
the external carotid. In addition to supplying the teeth, it is distributed
to the roof and floor of the mouth, to the maxillary sinus, and to other
parts of the face and head. It has its origin from the external carotid
artery opposite the condyle of the mandible within the substance of the
parotid gland, passes forward between the condyle of the jaw and the
sphenomaxillary ligament, from which point it passes obliquely upward
and forward between the external and internal pterygoid muscles until
it reaches the sphenomaxillary fossa, where its terminal branches are
given off. It is divided into three portions — the first or maxillary,
the second or pterygoid, and the third or sphenomaxillary. The teeth
are supplied from branches of the first and third divisions, the upper
teeth receiving their blood-supply from the alveolar or superior maxillary
and the infra-orbital branches of the third division, while the lower
teeth are supplied by the inferior dental or mandibular branch of the first
division.

The alveolar or superior maxillary branch arises, in common
with the infra-orbital branch, from the internal maxillary as it passes into
the sphenomaxillary fossa. It passes downward, in a tortuous manner,
in a groove provided for it in the back of the maxilla. In its downward
course it gives off the following branches: The antral, to supply the an-
trum; the dental (known as the posterior dental arteries), which pass into
the substance of the bone through the posterior dental canals to supply
the molar and bicuspid teeth ; the alveolar or gingival, to supply the gums;
and the buccal, to the lateral walls of the mouth. The anterior upper
teeth are supplied through the infra-orbital branch of the internal maxil-

9 1

Fig. 50. — Dissection Showing Blood-supply to the Teeth.

THE BLOOD-SUPPLY TO THE TEETH.

lary\ This branch arises from the internal maxillary artery, generally,
in common with the posterior dental. It passes forward in company
with the maxillary division of the fifth nerve — first along the groove and
then in the canal on the orbital plate of the maxilla, and finally makes
its exit upon the face through the infra-orbital foramen. Besides giving
off branches to the orbital and nasal cavities, it supplies the incisor and
cuspid teeth through its anterior dental branch, which passes downward
through a groove in the anterior wall of the maxilla.

Infraorbital artery and nerve

Spheno-pa/afine branch

Posterior or descending palatine branch

Auso-j.uiattnc brunt h

Vidian branch

interior deep temporal artery
External pterygoid branch

Orbital branch

Palpebral

branch

Xusat branch

dental branch

Labial branch

Posterior dental

branch

Alveolar branch

Incisive branch
Mental branch

Submental branch

Posterior deep temporal inter)/

Small meningeal
artery

Middle meningeal
artery

Temporal ariern
Tom panic branch

Deep auricular

brunch
A URICULO-TE3I-
PORAL KERVE

Masseteric branch

External carotid
a iter if

Interned lateral or
spheno-mandibu-
lar ligament

Mnndihulnr or
inferior dentul
artery and nerve

Buccal branch with Internal pterygoid branch

portion of buccal nerve

Mylo-hyoidean branch
Fig. 51. — Scheme of Internal Maxillary Artery. (Morris.)

The Inferior Dental Artery (Fig. 51). — The lower teeth receive
their blood-supply through the inferior dental or mandibular artery.
This artery arises from the under part of the internal maxillary as it
passes downward and forward between the sphenomaxillary ligament
and the neck of the jaw and enters the inferior dental canal through the
inferior dental foramen. It passes forward in the canal accompanied
by the inferior dental nerve, and in so doing sends off twigs to supply

Fig. 52. — Dissection Showing Nerve-supply to the Teeth.

THE NERVE-SUPPLY TO THE TEETH. 95

the molar and bicuspid teeth. When the mental foramen is reached,
it divides into two branches, the incisive branch and the mental branch.

The incisive branch continues its course within the cancellated struc-
ture of the bone, sending off minute branches which supply the anterior
teeth, the terminal branches anastomosing with the artery of the opposite
side.

The mental branch passes out through the mental foramen accom-
panied by the mental branches of the inferior dental nerve, and supplies
the tissues of the chin and lower lip.

The Veins. — The blood, in returning from the teeth to the heart,
is first taken up by the posterior dental and inferior dental veins, which
in their course follow closely that of their corresponding arteries. These
veins, in conjunction with others which accompany branches of the inter-
nal maxillary artery, form the pterygoid plexus. At the posterior con-
fluence of this plexus the returning blood empties into the internal maxil-
lary vein. Accompanied by the internal maxillary artery it passes back-
ward and outward, enters the parotid gland, and finally empties into the
temporomaxillary vein midway between the zygoma and the angle of the
jaw. After leaving the substance of the parotid gland, the temporo-
maxillary vein passes downward until near the angle of the jaw, where
it divides into two branches, one of which passes downward and slightly
forward, uniting with the facial to form the common facial vein,
and the other, after passing downward and backward, empties into the
external jugular vein. The external jugular vein returns the principa
portion of the blood from the teeth, and from its point of beginning it
passes almost perpendicularly downward and empties into the sub-
clavian vein, which, by joining with the internal jugular vein, forms the
innominate vein, which, in turn, empties into the superior vena cava,

thus communicating with the heart.

THE NERVE-SUPPLY TO THE TEETH.

The nerves supplying the teeth are derived from branches of the
fifth cranial nerve, otherwise known as the trifacial or trigeminal nerve.
The fifth nerve is the largest of the cranial nerves, and consists of two parts,
a large root (sensory) and a small root (motor). The larger portion passes
into a ganglion (the Gasserian ganglion), frequently compared to the
ganglion on the posterior root of the spinal nerve. It arises, or makes its
appearance, at the surface of the brain, on the anterior part of the side
of the pons Varolii. The sensory root, which, through its branches,

9 6

ANATOMY.

supplies the teeth, is composed of from 80 to 100 filaments, each inclosed
in a neurilemma, the entire bundle being bound together in a single nerve.

The fifth nerve is divided into three divisions: First, or ophthalmic;
second, or superior maxillary; and third, or inferior maxillary (mandibu-
lar). The branches which supply the teeth are included in the second and
third divisions, the upper teeth being supplied by branches from the
superior maxillary nerve, and the lower teeth by branches from the in-
ferior maxillary nerve.

The Second Division, or Superior Maxillary Nerve (Fig. 53).
This nerve, composed entirely of sensory fibers, is intermediate in size

AXTERWR VEXTAL MAXILLARY XERYE ORBITAL BRANCH

VAX1LLARY

AT/;

ME< 'EEL'S GANGLION

POSTERIOR DENTAL

LOOP FORMED BY MIDDLE AXD ANTERIOR DENTAL NERVES
Fig. 53. — The Maxillary Nerve, seen from Without. {Morris, after Beaunis.)

between the inferior maxillary and the ophthalmic divisions. It passes
forward from the Gasserian ganglion and leaves the cranium through the
foramen rotundum. It traverses the upper part of the sphenomaxillary
fossa, and passes into the orbit through the sphenomaxillary fissure; then
passes forward along the infra-orbital groove, and enters the infra-
orbital canal, where it receives the name of the infra-orbital nerve. Pass-
ing through this canal, it emerges upon the face through the infra-orbital
foramen. The superior maxillary nerve, beside supplying the teeth,
sends off branches to the dura mater, to the orbit, and terminal branches
in three groups — labial, nasal, and palpebral. The branches given off

THE NERVE-SUPPLY TO THE TEETH.

to the teeth are the posterior superior dental, the middle superior dental,
and the anterior superior dental.

The posterior superior dental arises from the second division of the
fifth nerve, by one or two roots, just before it passes into the infra-orbital
canal. It is divided into a superior and an inferior set; the former passes
forward and terminates in the canine fossa, while the latter, usually the
larger, enters the posterior dental canals, and, following the line of the

Fig. 54. — Dissection showing Mandibular (Third) Division of Fifth Nerve, a, Temporal
Bone; b. Mental Branches emerging through Mental Foramen; c, Lingual Nerve; d, Man-
dibular or Inferior Dental Nerve.

alveolar process through minute canals in the bone, sends off twigs to the
molar teeth, ending in a plexiform manner by communicating with the
middle superior dental nerve. This nerve is also distributed to the gums
and adjacent buccal mucous membrane.

Middle Superior Denial Nerve. — The infra-orbital nerve, soon after
entering its canal, gives off this branch, which passes outward, downward,
and forward over the outer wall of the maxillary sinus, and, after forming
7

98 ANATOMY.

plexuses with the posterior dental branches, gives off filaments to supply
the bicuspid teeth.

The anterior superior dental nerve, which is the largest of the dental
set, is given off from the infra-orbital nerve, enters a canal close to the
infra-orbital foramen, passes over the anterior wall of the maxillary
sinus, and, after communicating with the middle and posterior dental
nerves, divides into ascending and descending branches, the latter being
distributed to the incisor and cuspid teeth.

The Third Division, or Inferior Maxillary Nerve (Fig. 54). —
This is the largest of the three divisions of the fifth nerve, and is both
motor and sensory in its function. Besides being distributed to the
lower teeth, it sends filaments to the lower portion of the face, the muscles
of mastication, the tongue, and mandible. It arises from the Gasserian
ganglion, passes downward, and emerges from the skull through the fora-
men ovale, after which it divides into a small anterior (motor) branch
and a large posterior (sensory) branch.

The Inferior Denial Nerve. — This is the largest branch of the inferior
maxillary nerve. From its point of origin it passes downward internally to
the external pterygoid muscle, and, upon reaching a point between the
ramus of the mandible and the sphenomandibular ligament, it enters
the inferior dental canal through the posterior or inferior dental foramen.
Before entering the foramen, two branches are given off, a lingual and a
mylohyoid branch. The nerve is accompanied through the inferior
dental canal by the inferior dental artery, and, when the mental foramen
is reached, it terminates by dividing into an incisive and a mental branch.
Between the posterior dental foramen and the mental foramen the nerve
gives off a series of twigs to the bicuspid and molar teeth, and these, by
communicating with one another within the substance of the bone, form
a fine plexus.

The incisive branch follows the incisive arteries through the sub-
stance of that part of the bone between the mental foramen and the
symphysis, and supplies the incisor and bicuspid teeth, while the mental
branch passes forward to supply the chin and lower lip.

CHAPTER VII.

Other Structures Within the Mouth.— The Gums.
Membrane. — The Alveolodental Membrane.
Glands, Ducts, Etc.

-The Mucous

The Gums (Gingiva). — The gums are formed by a layer of tough
fibrous vascular tissue, covering the alveoli, closely attached to their
periosteum, and provided with a free margin (gingival margin), which

Compact
Bone

Epithelium

Fig. 55. — Section through Gum and Alveolus.

is closely molded to the necks of the teeth. They are covered on both
aspects by the general mucous membrane of the mouth, that overlying
the labial and buccal surfaces being reflected from the lips and cheeks,

IOO ANATOMY.

the palatal surface being continuous with that of the hard palate, and
the lingual surface reflected from the under surface of the tongue and
floor of the mouth. In the immediate region of the necks of the teeth the
gums are especially thin and hard, being closely adherent to the peri-
osteum and alveolodental membrane in this region. In passing toward
the base of the alveolar walls the tissue becomes less firmly attached to
the underlying structure, and, when finally passing into the mucous
membrane of the cheeks and lips, is quite loose and flabby. This condi-
tion also prevails on the lingual aspect, but the palatal surface remains
firm throughout, the entire mucous membrane overlying the roof of the
mouth being similar in structure and attachment to that portion imme-
diately surrounding the necks of the teeth. In various situations about
the labial, buccal, and lingual surfaces of the gums small slender folds
of mucous membrane are found extending to the surrounding tissues.
These folds, which act as a bridle or curb to the adjacent movable parts,
are known as the frena of the mouth. The principal frena are found at
the median line, and are three in number — the frenum labium superioris,
frenum labium inferioris, and the frenum linguae. The two former
extend from the inner surface of the lips to the gums, to which their extent
of attachment is somewhat variable. The frenum labium superioris is
usually much larger than the frenum labium inferioris, and its attachment
to the gum frequently extends almost to the gingival border. The frenum
lingua; extends from the under surface of the tip of the tongue to the lin-
gual surface of the inferior gums. This is a much stronger fold than
those connected with the lips. Similar bridles are found in the buccal
region, usually near the bicuspid teeth, but they are much smaller than
those at the median line. The gingival margins, or that portion of the
gums embracing the necks of the teeth, present much variety in outline.
Instead of encircling the neck of the tooth in a direct line, the margins
are made up of a series of semicircles. Using the incisive region for
reference, the labial and palatal margins are concave rootward, while the
interproximate spaces are partly or completely filled by gum tissue,
having the outline reversed or convex in the direction of the crowns of
the teeth. The gingival margin is also termed the "free margin of the
gum," this name better describing its extent. As previously stated, the
gums are attached to the periosteum and peridental membrane, but in
most instances, and particularly before the adult period, the free margins
of the gums extend beyond the alveolodental membrane, the limit of which
is formed by the cervical line. That portion of the gum margin beyond
the cervical line is in close contact with the neck of the tooth, but is not

MUCOUS MEMBRANE OF THE MOUTH. IOI

adherent to it, the connecting medium, the alveolodental membrane, not
being present to form the attachment. The curvature of the gingival
margins, and the nature of the tissues which enter into their construction,
are usually considered as strongly indicative of the temperament of the
individual. Thus, in the bilious temperament the margins are inclined
to angularity and the tissues rather thick and firm. In the sanguine type
the outline formed is almost a perfect semicircle, and the tissues are of
moderate thickness and firmness. In the nervous type the curvature is
strongly parabolic, and the tissues firm and delicate. In the lymphatic
the tissues are loose and thick, and the curvature is long and poorly
defined. In some instances the interproximate spaces are completely
filled by the gingivae; in others the space is only partly occupied by these
tissues. The former condition is present when the proximate surfaces of
the teeth are nearly or quite parallel with each other, thus reducing the
capacity of the space. The latter condition is present when the crowns
of the teeth are bell-shaped and the interproximate spaces extensive. The
labial and buccal surfaces of the gums are more or less broken by numer-
ous prominences and depressions, all of which accord with the variations
upon the surface of the bone beneath.

Some distinction should be made between the gingival line and the
cervical line, the former referring to the free margin of the gum, and not a
fixed line, while the latter refers to that positive line established on the
tooth by the union of the enamel and cementum.

Mucous Membrane of the Mouth. — The term "membrane"
in a general sense is one applied to thin layers of tissue, somewhat elastic
and of a whitish or reddish color. Such tissues are found lining either
closed cavities or canals which open externally, absorbing or secreting
fluids, and enveloping various organs. The simple membranes are of
three varieties, being either mucous, serous, or fibrous. The mucous
membranes are so called from the clear viscid fluid (mucus) which they
secrete. They line the various cavities or tracts of the body which com-
municate with the exterior. The three grand divisions of mucous mem-
brane are those lining the digestive, respiratory, and genito-urinary pas-
sages. Lining the entire cavity of the mouth we find the beginning of the
digestive tract, being continuous with, in many respects similar to, the skin
on the exterior and performing similar functions within. It is soft, smooth,
and velvety, of a bright red color, and quite vascular; it is covered on the
exterior by a layer of epithelial cells overlying the vascular parts. Imme-
diately beneath this is a network of fibrous connective tissue forming the
proper mucous membrane, and still deeper is a third layer, somewhat

102 ANATOMY.

loose in texture, but composed of fibrous connective tissue, the submucous
membrane. The oral mucous membrane, at its point of beginning on the
contiguous surfaces of the lips, is endowed with keen sensibility; it is dry,
bright red in color, and plentifully supplied with vascular papilla', in
many of which are sensory nerve terminals. Distributed along the line
of junction between the integument and the mucous membrane are
numerous sebaceous follicles, which, however, are devoid of hair-bulbs.
The characteristic dryness of this surface gradually becomes changed to a
mucus-secreting one, as that part of the membrane lining the interior of
the lips is approached. Distributed over the surface of the labial mucous
membrane are a number of minute openings, the mouths of the labial
glands, which lie immediately beneath the membrane. The buccal mu-
cous membrane, or that lining the cheeks, is similar to that covering the
internal surface of the lips. It is penetrated at various points by the
mouths of the buccal glands, which, in general, are smaller and less
numerous than the labial glands. In the region of the second molar
teeth the membrane is broken by four or five openings of larger size,
which communicate with the molar glands. The mucous membrane
covering the hard palate is thick and firm, less brilliant in color than that
covering the cheeks and lips, and firmly bound down to the periosteum.
Running from before backward at the median line, the membrane is
formed into a slight fold, the median raphe, while near the anterior portion
of the palate are a number of fantastically arranged folds, the ruga (see
General Description of the Mouth). The thin but rather dense fibrous
aponeurosis forming the soft palate is covered anteriorly by the oral mu-
cous membrane. Suspended from the centerof the free margin of the soft
palate is the uvula, which is likewise covered by mucous membrane, and
from the base of this, on either side, are two muscular folds, which extend
outward and downward, forming the anterior and posterior pillars of the
fauces. The mucous membrane covering the tongue has already been
described in connection with that organ. From the mouth the digestive
mucous membrane passes through the fauces, pharynx, and esophagus
to the stomach, and is so continued throughout the whole digestive
tract. Other prolongations also pass into the ducts of the salivary
glands.*

The Alveolodental or Peridental Membrane.

This membrane invests the roots of the teeth, and at the same time
lines the wall of the alveoli. Being reflected from the periosteum cover-

* For a minute description of the mucous membrane of the mouth see Part ii.

THE ALVEOLODENTAL PERIOSTEUM OR ROOT MEMBRANE.

IO3

ing the outer alveolar walls, it enters the alveolar sockets as a single
membrane, affording nourishment to the bone on one side and to the
cementum of the tooth on the other. It is a connective tissue of moderate
density, and is rich in its nerve- and blood-supply. The general direction
of its fibers is transverse, being attachedot are fully
outlined. Between the seventh and eighth year the cutting-edge of the
tooth begins to make its appearance through the gum at a point either to
the rightfor left ot the median line, and, by a gradual absorption of the

lie

UPPER CENTRAL INCISOR.

Ill

gum tissue, eruption takes place. During the following year about one-
eighth of an inch has been added to the length of the root. At the end of
the eighth year the root has become calcified to about one-half of its
completed length. During the ninth year, owing to a reduction in the
diameter of the root, the extent of growth has almost doubled that of
the previous year, and a decided narrowing of the free root margins is
to be observed. At the eleventh year calcification is completed in the
outer root walls, and may be said to be completed (Fig. 58).

The Crown of the Upper Central Incisor presents for examination
four surfaces — labial, lingual, mesial, and distal; two angles — a mesial
and a distal; and a cutting-edge. The
general form of the crown is that of a
double inclined plane, or wedge-shape,
the cutting-edge representing the junc-
tion of the two sides of the incline, one
of which looks anteriorly (labial) and
the other posteriorly (lingual). The
labial side of the incline is convex,
while the lingual is concave from the
cutting-edge toward the root; but,
upon reaching its upper or cervical
third, it presents a slight general con-
vexity. The base of the wedge is
directed upward and partakes of the
contour of the neck of the tooth. Fig. 59.

The Labial Surface of the
Crown (Fig. 59). — In general outline this surface resembles an imper-
fect quadrilateral. The margins of the surface are the mesial, the distal,
the cervical, and the incisive. The mesial margin begins at the lower
border or cutting-edge and passes upward, usually with a slight distal in-
clination, gradually uniting with the cervical margin. The distal margin
begins at the cutting-edge and passes upward with a slight mesial
inclination, also joining the cervical margin. Both of these margins
possess more or less general convexity, and, at their junction with the
cutting-edge, form the mesial and distal angles of the crown. The
cervical margin is rounded and gradually passes into the two lateral
margins just described. The incisive margin is marked by the cutting-
edge, and extends from the mesial angle on one side to the distal
angle on the other. These four margins, which assist in giving to the
tooth its typal form, are quite variable. This difference particularly

Cervical
Ridge

Mesia
Angle

Cutting Labial
Edge Grooves

—Upper Central Incisor,

Labial Surface.

112

ANATOMY.

marked on the mesial and distal margins, where, in some cases, there
is a decided convergence in the direction of the root, forming what is
commonly termed the bell-shaped crown, while in others the same
margins will be nearly parallel with each other, making the width of the
crown almost as great at the cervical margin as at the cutting-edge. The
mesial angle is usually pointed and square, while the distal is much rounded.
This surface of the crown is slightly convex from above downward, as well
as from side to side, and in the majority of instances is of greater vertical
than transverse extent. Beginning at the incisive margin are two slight

longitudinal depressions or grooves —
the labial grooves — which are resultant
from the developmental lobes pre-
viously composing the primitive cutting-
edge, and, for this reason, are otherwise
known as developmental grooves. In
many instances one or more transverse
ridges are found upon the cervical por-
tion, but these are supplemental in
character.

The Lingual Surface of the
Crown (Fig. 60). — This surface has its
borders formed by three marginal
ridges and the cutting-edge. The
marginal ridges are pronounced eleva-
tions of enamel, and surround the
surface upon three sides, the intervening space in many instances
being a decided concavity or fossa — the lingual fossa. The mesial
marginal ridge begins at the mesial angle of the crown, passes upward,
inward and backward, following the curvature of the mesial border. The
distal marginal ridge begins at the distal angle of the crown in a somewhat
less pronounced form, passes upward, backward, and inward, following
the curvature of the distal surface. Upon reaching the cervical portion
of the crown these two margins unite and form the cervicomarginal ridge.
This ridge may be bold and prominent, or it may be but slightly developed.
Near its center it is frequently broken by a depression or pit — the lingual
pit. In some instances this pit is deeply penetrating; in others it assumes
the form of a fissure, and may completely sever the ridge. This border
is sometimes elevated into a slightly developed tubercle or cusp — the
cuspule. When this is present it has the appearance of being produced
by a fold of enamel, and encircling it is a well-marked fissure. The

Cervical
Ridge
Distal
Marginal
Ridge

Distal
Angle

Cervical
Line

Mesial

Marginal

Ridge

Mesial
Angle

Lingual Fossa

Fig. 60. — Upper Central Incisor,
Lingual Surface.

UPPER CENTRAL INCISOR.

J1 3

Ceivica]
Line

lingual fossa is usually traversed by two longitudinal grooves, which
correspond to the developmental grooves of the labial surface. When
a cuspule is present, the fissure which surrounds it frequently passes
into the fossa, or the fossa may be partly covered by the cuspule over-
hanging its cervical portion. When the cervicolingual fissure exists, it is
not unusual for it to bifurcate and throw a branch along the inner border
of each marginal ridge, or it may penetrate the fossa proper and divide
it into two parts. The lingual surface is somewhat less in extent than the
labial surface, this reduction being principally in a mediodistal direction,
the length of the two surfaces from the
cutting-edge to the cervical margin
being about equal.

The Mesial Surface of the
Crown (Fig. 61). — The outline of this
surface resembles an inverted cone or
triangle, the lines of which are more
or less broken, the apex of the cone
terminating at the cutting-edge and
the base directed toward the root of
the tooth. The base of the cone is
made concave by the enamel margin
or cervical line. The margins of the
mesial surface are the labial, the
lingual, and the cervical. The labial
margin is convex and rounded through-
out its entire extent, from the cutting-edge to the cervical line. The contour
of this margin varies with the typal form of the crown, in some presenting a
decided and well-marked convexity, in others being but slightly curved.
The lingual margin is concave and rounded, but the line is much broken.
Beginning at the cutting-edge, it is decided and square, this feature
usually including the lower third. As it passes upward and the center
is approached, the line is more concave and rounded in a mesiolingual
direction, this latter feature increasing upon approaching the cervical
line. The cervical margin is that formed by the cervical line. It is
usually well-defined, being concave or V-shaped, with the point of the V
more or less rounded, and with its free ends pointing one in a labial and
one in a lingual direction, the former being a trifle longer than the latter.
The surface between the borders presents a slight general convexity,
but with an inclination to flatness near the cervical portion, which is
occasionally developed into a slight concavity. Whatever deviations

Ceivico-
lingual Ridge

Mesial Angle

Fig. 61. — Upper Central Incisor,
Mesial Surface.

ANATOMY.

Lingual
Fossa

Lingual
Ridge

1- !(,. <)2

may be present in the borders of this surface, from those assumed in the

description just given, their union at the cutting-edge will always be in

a direct line with the long axis of
the tooth.

The Distal Surface of the
Crown (Fig. 62) . — In a general way,
this surface resembles the mesial
surface just described. There are,
however, one or two minor points
of distinction: the borders are all
more rounded, the labial border
presenting a greater convexity, and
the lingual a more perfectly formed
concavity. The surface is quite
full in the center, from which it
slopes away in all directions, thus
producing a decided general con-
vexity. The cervical margin of the

surface is almost identical with the cervical margin of the mesial surface.

The distance in a direct line from the cervical border to the cutting-edge

is a trifle less than the corresponding measure-
ment on the mesial surface. The distal angle

is equally constant in its position, and, being

connected with the mesial angle in a direct

line by the cutting-edge, finds this latter margin

always in the labiolingual center of the crown.
71ie Cutting-edge of the Central Incisor. —

The cutting or incisive edge receives its name

from its function, that of cutting or inci>in<^

the food. It is formed by the junction of the

labial and lingual surfaces of the crown, and

extends almost in a direct line from the mesial

to the distal surface; at its union with the

mesial surface it assists in forming the mesial

angle of the crown, and serves the same pur-
pose by its union with the distal surface. In

the majority of instances it is an unbroken Surface showing Develop-

■> J mental Grooves — a

line. In passing from the mesial to the distal

angle it converges slightly in the direction of the root, thus making the

crown a trifle shorter on the mesial than on the distal side. In the

Fig. 63. — A Young Upper
Central Incisor, Labial

UPPER CENTRAL INCISOR. 115

recently erupted tooth (Fig. 63) the line is broken by the developmental
grooves; these usually disappear by wear, but occasionally traces of their
existence remain, and thus permanently break the positive line that
would otherwise be present. As the cutting-edge approaches the distal
angle of the crown it is inclined to slope away, producing a less positive
angle than the corresponding mesial angle. In some instances the
cutting-edge is quite thin and inclined to sharpness, in others it is
blunt and dull, the former condition being present when there is a
decided overbite in occlusion, the latter occurring when this feature is
less pronounced. The cutting-edge is frequently referred to as the
occlusal surface, this term being employed to make the description more
uniform with the bicuspids and molars, and for this reason is permissi-
ble; but it is only in rare instances that the surfaces occlude directly with
the opposing teeth, the condition being most frequent in teeth of the
lymphatic type, and in cases of malocclusion.

The Cervical Margin. — This margin, which is distinctly outlined
by the free extremity of the enamel covering of the crown, also marks
the extent of the membrane covering the root. The margins formed by
this line are those of a double concavity and a double convexity. On
the labial and lingual portions it is concave rootward, while on the
mesial and distal sides it is convex in this direction. If a line be drawn
around the tooth at the extreme upper point of the enamel covering,
it will be found to touch only the labial and lingual prolongations, while
a space will exist between the line thus drawn and the cervical margins
of the proximate surfaces. The character of the cervical curvature
varies with the type of the tooth, being more or less pronounced as the
case may be. In a typical central incisor the cervical line of the labial
surface will usually form the segment of a larger circle than that of the
lingual, and, while the mesial convexity may be gracefully curved, the
distal may incline to angularity.

The Neck of the Tooth. — The neck of this tooth partakes of a form
between that of the crown and root which it joins. It is principally
formed by a sudden sloping of the enamel margin to meet the root. It
is broader on the labial than on the lingual surface, and is somewhat
flattened laterally, with an occasional depression or concavity on its
mesial portion. The neck of this tooth is seldom a decided anatomic
feature, being less pronounced than upon any other tooth. In the bicus-
pids and molars both the crowns and the roots assist in forming the neck
by a constriction of their adjacent parts, while in this tooth the crown
alone is instrumental in this direction.

Il6 ANATOMY.

The Root of the Upper Central Incisor. — The root of this tooth
is conic in form, its base directed downward, its apex upward. Viewed
in transverse sections its outline is that of a rounded triangle, one side
of which faces in a labial, one in a mesiolingual, and one in a distolingual
direction. The labial side is the most flattened, while the two remaining
sides are of equal length and oval in form. This triangular outline
usually continues throughout the entire length of the root, but in some
instances, near the apical end, may have a decided or slight distal curve,
included in which will be a more circular form. The taper of the root
from the base to the apex is very gradual upon the labial and lingual
surfaces, until the apical third is reached, when the two sides converge
more rapidly. The mesial and distal surfaces are somewhat flattened
and taper very gradually from the base to the apex. In a majority of
instances the root is much longer than the crown, but in rare cases its
length is barely equal to, or less than, that of the crown.

Bilious Type. — The crown of the upper central incisor in this
type is of greater longitudinal than transverse extent; large in size, abound-
ing in angles rather than curves. It possesses neither brilliancy nor
transparency of surface, but is slightly inclined to translucency. The
labial surface is flat, with more or less decided transverse ridges in the
cervical portion. The labial grooves are generally present in the form
of well-defined depressions. On account of the angular nature of the
tooth, this surface approaches closely to the quadrilateral form. The
mesial and distal surfaces are flat, with their margins bold and well
defined. The lingual surface also shows the angular nature of the crown
in having its marginal ridges squarely set and its developmental grooves
definitely outlined. The cutting-edge is rather thin, square, and sharp,
the line frequently being imperfectly formed. The mesial and distal
angles are both well produced, and the cervical margin, in keeping with
the rest of the parts, is inclined to angularity.

Nervous Type. — The central incisor common to this temperament
is delicate and graceful in outline. The crown is of medium size, with
the length predominating over breadth. The enamel is inclined to trans-
parency, and is of a blue or bluish-gray color, presenting much brilliancy.
The labial surface is fairly well rounded, and the labial grooves are
present as slightly rounded depressions, which frequently extend well
toward the cervical margin, where they gradually disappear. In general
outline this surface partakes of the triangular form, the crown of the
tooth being broad at the cutting-edge and much constricted at the neck.
The mesial and distal surfaces show a convexity in every direction,

UPPER CENTRAL INCISOR. 117

and the nature of the occlusion is manifest from the decided wedge-
shape appearance of the crown providing for a long overbite. Upon
the lingual surface but little in the way of detail is to be observed, the
entire surface from the cutting-edge to the cervical ridge being smooth
and concave. The cuspule previously referred to is occasionally present
in this type, breaking the general smoothness of the surface with its
prominence. The marginal ridges are poorly defined; the cutting-edge
is a sharp, unbroken line;- the mesial and distal angles are present in
the form of long, graceful curves, rather than definite angles, this being
particularly true of the distal angle. The cervical line is decidedly
curved, the labial and lingual portions being deeply concave rootward,
while the mesial and distal are decidedly convex.

Sanguineous Type. — The crown of the central incisor is usually
above the average in size, but is well proportioned, abounding in curves
and rounded outlines. The enamel is inclined to translucency, particu-
larly near the cutting-edge. The labial surface is smooth and rounded;
the depressions formed by the labial grooves are slightly observable,
and extend but a short distance from the cutting-edge. The surface
is somewhat greater in longitudinal than in transverse extent, and ap-
proaches much nearer to a circular form than the corresponding tooth
of other types. The mesial and distal surfaces are well rounded, making
the point of contact with approximating teeth near the center of the
surface. The lingual surface abounds in heavy rounded lines; the
marginal and cervical ridges are particularly prominent, diminishing
the extent of the lingual fossa. A cuspule is frequently present in the
form of a well-rounded prominence. The cutting-edge is of moderate
thickness and slopes away from the center in either direction to assist
in forming the rounded mesial and distal angles. The cervical curvature
on the labial and lingual surfaces is an unbroken semicircle, while that
of the mesial and distal surfaces is less uniform.

Lymphatic Type. — In the central incisor of this typal form the
crown is large, but not shapely, and the breadth is equal to, or exceeds,
the length. The enamel coloring is muddy or brownish-yellow, and the
surface is lacking in brilliancy. The labial surface is flat and smooth,
with a faint sign of the labial grooves. The general outline of this
surface is that of a circular cone, with the cutting-edge for the base, and
the apex formed on the cervical margin. The mesial and distal aspects
present a striking contrast to the types previously described, by having
a labiolingual diameter greater than that represented between the cervical
line and the cutting-edge. These two surfaces are convex in a labio-

Il8 ANATOMY.

lingual direction only, making the point of contact with approximating
teeth an extended surface rather than a single point.

The lingual surface is heavy and bulky, frequently to such a degree
as to produce a general convexity rather than a concavity, as found in
most typal forms. This surface is frequently broken by one or more
longitudinal grooves, but is seldom crossed by transverse lines of any
kind. The cutting-edge is barely deserving of the name. Although
formed by the free borders of the labial and lingual surfaces, these two
planes are so far separated at their incisive margins that the space between
them, instead of being an edge, becomes a more or less broadened surface,
and one upon which the lower incisors frequently occlude. The line
thus formed is straight and direct from the mesial to the distal angle
of the crown, both of which are well produced. The cervical curvature
is represented by the segment of a much larger circle than that found
upon teeth of other types, and the neck of the tooth is heavy and bulky,
showing but little constriction at this point.

UPPER LATERAL INCISOR.

jth

5th

7th

9 th

ioth

i ith

year

Fig. 64.

Calcification' Begins, from Three Centers, First Year after Birth.
Calcification Completed, Tenth to Eleventh Year.
Erupted, Seventh to Eighth Year.
Average Length of Crown, .34.

Average Length of Root, .51.

Average Length over All, .85.

Like the central incisor, calcification in this tooth begins during
the first year after birth, the process taking place in the same manner,
from three centers, along the future cutting-edge, and gradually extend-
ing in the direction of the root. By the expiration of the third year the
cutting-edge and the angles of the crown are fully formed; the fourth
year finds the crown calcified to nearly one-half its completed length;

UPPER LATERAL INCISOR.

II 9

by the fifth year the cervical ridge is reached; while the sixth year usually
completes the process of coronal calcification. At the close of the seventh
year the base of the root is fully outlined, and during the following year
about one-eighth of an inch is added to its length, and still greater progress
is made during the ninth year, by which time fully three-fourths of the
root length has become calcified. During the tenth year the apical end
of the root begins to form by a sudden doubling-over of the free cal-
cifying margins, and by the eleventh year the surface of the root is com-
plete (Fig. 64). By the above description it will be observed that at the
time of eruption the root of this tooth is only calcified to about one-half
of its completed length, and the same may be said of the central incisor;
but so much time elapses between the beginning of the eruptive stage
and the period at which this phenomenon is completed that the calcifica-
tion of the root is usually finished by the
time the tooth assumes its permanent
position in the jaw.

The crown of the upper lateral
incisor, like that of the upper central
incisor, presents for examination four
surfaces — labial, lingual, mesial, and
distal — a cervical margin, a cutting-edge,
and a mesial and distal angle. The
general contour of the crown closely re-
sembles that of the upper central incisor,
except that it measures about one-third
less from mesial to distal, and is a trifle
shorter from the cutting-edge to the
cervical line. As in the central incisor,
the labial and distal surfaces form a
double incline plane, and unite below to

form the cutting-edge. The labial side of the incline is convex, while
the lingual is concave, but seldom so marked as that found upon the
central incisor. The base of the wedge, or double incline, formed by
the cervical margin, is correspondingly smaller than that of the crown of
the central incisor.

The Labial Surface of the Crown (Fig. 65). — This surface of the
crown of the upper lateral incisor is more irregular in outline than the
corresponding surface of the central incisor. The margins of the surface
are the mesial, distal, cervical, and incisive. The mesial margin begins
at the mesial angle and passes upward with a decided distal inclination

Distal
Angle

Cervico-
labial
Ridge

Mesial
Angle

Labial Grooves

Fig. 65. — Upper Lateral Incisor,
Labial Surface.

120

ANATOMY.

Lingual

Ridge

Mesial Mar

ginal Ridge

Cervical
Ridge

Distal Mar-
ginal Ridge

to meet the cervical margin. The distal margin is shorter and decidedly
more convex than the mesial margin, this variation in outline being still
more marked when compared with the corresponding margin of the
central incisor. At the cutting-edge these two margins assist in forming
the mesial and distal angles of the crown, and by their continuation and
union above form the cervical margin. The incisive margin is formed
by the cutting-edge. Like the central incisor, the four margins of this
surface vary greatly in the different types; this is particularly true of
the two lateral margins, which at times are found to be in the form of a

direct line, or even slightly concave,
while in others they are both decid-
edly convex. This surface of the
crown shows a greater general con-
vexity than the labial surface of the
central incisor, the cervical portion
presenting a curve much more de-
cided than that near the cutting-
edge. The labial grooves are in all
respects similar to those described in
connection with the central incisor,
and extend from the cutting-edge
toward the center of the surface, where
they gradually disappear. Trans-
verse ridges are occasionally found
near the cervical portion of the surface.
The Lingual Surface of the Crown (Fig. 66). — This surface of
the upper lateral incisor is subject to much variation in form, but presents
the same points for examination as the corresponding surface of the
central incisor. These consist of the marginal ridges, which are usually
more pronounced than those of the central, making the concavity or
fossa between them small and deep. In some instances the surface will
be smooth and flat, with an entire absence of ridges or fossa.'. The
distal marginal ridge is shorter and more bowed than the mesial, and the
cervical ridge is well marked and proportionately broader and stronger
than in the central incisor. In some instances the marginal ridges are
but slightly developed, with their cervical ends broadened and separated
by a deep fissure, giving the appearance of a terminal fold in the enamel.
The cervical ridge is frequently broken by a cuspule, which is usually
more pronounced than when found upon the central incisor. The
lingual fossa may be present as a smooth, unbroken concavity, or it may

Lingual Grooves

Fig. 66. — Upper Lateral Incisor,
Lingual Surface.

UPPER LATERAL INCISOR.

121

1^ ig. 07. — Upper
Lateral Incisor,
Mesial Surface.

be subdivided by a longitudinal ridge, which often exists to such an
extent as to force the remaining portions of the fossa well against the
marginal ridges, where they will be observed as slight depressions rather
than marked concavities.

The Mesial Surface of the Crown (Fig. 67).
— Viewing the crown from this aspect, the outline
is that of an inverted cone or triangle. The lingual
margin of the surface is well defined, and the angle
formed by the union of this surface with the lingual
surface is moderately acute. The labial margin is
well rounded, and passes into the labial surface
without a decided line of demarcation. The sur-
face on its upper or cervical third is usually flat-
tened and occasionally concave. At the center, and
continuing toward the cutting-edge, it is decidedly
convex in every direction, thus producing a promi-
nent point of contact with the distal surface of the
central incisor.

The Distal Surface of the Crown (Fig. 68).
— This surface also shows the characteristic wedge-shape of the crown,
and is principally different from the mesial surface in being convex
throughout. Near the center it is well rounded and full, providing a
point of contact for the mesial surface of the cuspid.
The lingual margin, while being more decidedly out-
lined than the labial, is much more rounded than
the lingual margin of the mesial surface. From the
most prominent point near its center the surface slopes
away in every direction, the convexity being most
marked near the cutting-edge.

The Cutting-edge of the Lateral Incisor. — In the
young tooth the cutting-edge presents the three little
tubercles common to all incisors, the grooves which
divide them passing up over the labial and lingual
surfaces and forming the labial and lingual grooves.
These soon disappear by wear, usually leaving the
cutting-edge in the form of a direct line and connecting
the two angles of the crown. Like the central incisor,
this margin of the crown may be thin and sharp, or it may be thick and
dull.

The Cervical Margin. — The line of demarcation between the crown

Fig. 68. — Upper
Lateral Incisor,
Distal Surface.

122 ANATOMY.

and the root of the tooth resembles so closely that described in connection
with the central incisor that it will only be necessary to mention one or
two characteristic differences. The lingual side of the line presents a
much smaller curve proportionately, and usually extends a little higher
in the direction of the root than that represented upon the labial portion.
The mesial and distal portions of the line dip well down, decreasing the
length of the crown on these surfaces; the margin on the former surface
is usually angular and V-shaped, while on the latter it is circular in form.

The Angles of the Crown. — The angles of the crown are the mesial
and the distal, and are formed in the same manner as the same angles of
the central incisor. The mesial angle is generally well produced, in most
instances being slightly acute; but when the cutting-edge is thin and
frail, the angle is frequently much obliterated by wear. That portion
of the crown of the upper lateral incisor which is usually referred to as
the distal angle is scarcely worthy of the name. It is usually present as
a long curve, which begins near the center of the cutting-edge and extends
well up on the distal surface. This characteristic outline is sometimes
so pronounced as to completely destroy the cutting-edge, the distal
surface being carried forward by a long curve ending in the mesial angle.

The Neck of the Tooth. In this tooth the neck is usually marked
by a constriction much more pronounced than that found in the central
incisor. On the labial and lingual surfaces it is principally formed by
a sudden sloping of the enamel surface rootward, but on the two lateral
surfaces it is formed by a flattening or slight concavity of both the crown
and the root.

The Root of the Upper Lateral Incisor. — The root of this tooth
is conic in form, and is much more flattened from mesial to distal than
the root of the central incisor. At its junction with the crown it is circu-
lar in form, the labial portion forming the segment of a larger circle than
the lingual, this feature being observed throughout its entire length. The
flattening of the mesial and distal sides begins immediately above the
neck, and gradually increases as the center of the root-length is approached,
where it often develops into a slight longitudinal depression. As the
apex of the root is approached, this longitudinal depression gradually
disappears, and the root again becomes circular in form. The thickness
of the root is about one-third greater from labial to lingual than from
mesial to distal, and, while it is generally classed as a straight root, it is
frequently provided with a pronounced distal curve near the apical
extremity. In some instances it is found with a double mesiodistal
curve.

UPPER LATERAL INCISOR. 1 23

Bilious Type. — In this type the lateral incisor is frequently poorly
developed, the cutting-edge and distal angle are wanting, the crown
being in the form of a single conic cusp, the distal surface meeting the
mesial at a point near the mesial angle. This form of crown might
be classed as one of malformation, but the fact that it most frequently
occurs in this temperament would appear to indicate a normal condition.
When the crown takes the form common to incisors, it is of greater
longitudinal than transverse extent, the angles are well produced, and
the mesial and distal surfaces are flat and almost parallel with each other.
The labial surface is flat and is frequently broken by transverse ridges
near the cervical portion. The lingual surface presents well-marked
outlines and margins, a cuspule is seldom present, and the lingual fossa
is well marked, but not deep. The cutting-edge is thin and sharp, to
provide for the overbite, which is rather long. The cervical border is
square and angular.

Nervous Type. — In this typal form the neck of the tooth is a pro-
nounced feature. The crown is long and narrow, the constriction
forming the neck beginning well down on the crown and extending over
the cervical line to the surface of the root. The labial surface is convex
in every direction and the labial grooves fairly well defined. The mesial
surface is convex near the center and cutting-edge, but often shows a
slight concavity on its cervical portion. The distal surface is rounded
and smooth. The lingual surface presents a general concavity, a cuspule
being more frequently present than in other types. The lingual fossa
is deep, and often extends beneath the cervicomarginal ridge in the form
of a circular fissure. The cutting-edge is thin and sharp, providing for
a long overbite; the mesial angle is pointed and well formed, while the
distal is usually much rounded. The cervical line is well arched, forming
the segment of a much smaller circle than that seen on the same tooth
of other temperaments.

Sanguineous Type. — The crown is well proportioned, with the
length slightly predominating over breadth, all the surfaces being more
or less rounded and smooth, showing the crown to be made up of curves
rather than angles. The labial surface presents a graceful convexity
throughout; the mesial and distal surfaces are both convex, with their
margins poorly defined. The lingual surface shows the rounded nature
of the crown in having its fossa and marginal ridges oval and blending
one into the other. The cutting-edge is moderately heavy and dull,
in keeping with the overbite, which is short. The cervical line is made
up of curves rather than angles.

124 ANATOMY.

Lymphatic Type. — In this type the crown is generally of greater
transverse than longitudinal extent. The neck is poorly produced, the
crown and root uniting without any marked constriction of the parts.
The labial surface is much flattened from mesial to distal, and but slightly
convex in the direction of the long axis of the tooth. The mesial and
distal surfaces are but little rounded and are nearly parallel with each
other, so that the contact with adjoining teeth becomes an extent of
surface rather than a single point. The lingual surface is convex above,
but as the cutting-edge is approached it becomes flat, but seldom con-
cave. The marginal ridges are not well shown and the lingual fossa is
but a slight depression. The angles of the crown are well produced and
the cutting-edge thick and blunt, this marginal surface frequently occlud-
ing directly upon the opposing lower teeth. The curvature of the cer-
vical line is that of a long circle.

.->

UPPER CUSPID.

5th

6th

7th

8th

9th

10th

nth

year

year
Fig. 69.

year

Calcification' Begins, from Three Centers, Third Year after Birth.
Calcification Completed, Twelfth to Thirteenth Year.
Krupted, Twelfth to Thirteenth Year.
Average Length of Crown, .37.

Average Length of Root, .68.

Average Length over All, 1.05.

About the third year after birth calcification begins in the central
lobe, which is gradually extended laterally, until, at the fourth year
it is met by the two lateral lobes, which are somewhat later in beginning,
and by the fifth year the three are united, the former eventually establish-
ing the single cusp of the tooth and the latter two the mesial and distal
angles. About the sixth year two-thirds of the crown is formed, and by
the seventh year the constriction which marks the beginning of the neck

UPPER CUSPID.

12:

of the tooth commences to make its appearance. Between the seventh
and eighth year calcification in the crown is completed and the cervical
line established, during the following year nearly one-quarter of an inch
is added to the length of the root, and by the beginning of the tenth year
the root is formed for fully two-thirds of its entire length. Between the
twelfth and thirteenth years or at the time of eruption, calcification is
completed in the root so far as its surface is concerned (Fig. 69). In
this latter particular the cuspid tooth differs from most of the others in
being completely calcified previous to, or about the time of, its eruption,
the eruptive process in this tooth, therefore, differs from that of the other
teeth, being fully calcified about the time it makes its appearance
through the gum tissue. To reach its final position in the arch the tooth
moves bodily downward, the bone filling in behind; while in the incisors,
bicuspids, and molars the free calcifying root-extremities remain nearly
stationary, the crowns being
forced downward as the lime
salts are deposited.

The crown of the upper
cuspid presents for examination
four surfaces — labial, lingual,
mesial, and distal — two margins
— the cervical margin and the
cutting-edge — and a mesial and
a distal angle. In general out-
line it is of the simplest form,
resembling the primitive cone-
shaped teeth of many fishes.
When viewed by looking directly
upon the mesial or distal surface,
the wedge-shape common to the
incisors is observed. The base
of the double incline is, however, much broader proportionately than the
corresponding measurement of the incisors. Looking at the crown from
a labial or lingual direction, its function, as both a penetrating and
incising organ, may be observed in the single cusp from which it derives
its name. The cusp, which is formed at the expense of the cutting-
edge, divides this latter margin into two distinct portions — the mesial
cutting-edge and the distal cutting-edge.

The Labial Surface of the Crown (Fig. 70). — The contour of
this surface is that of a broken circle more or less perfectly drawn. It

Cervica*
Ridge

Mesial
Angle

Mesial Cut-
ting-edge

'ervical
Line

Distal
Angle

Oistal Cut-
ting-edge

Labial Ridge
Fig. 70. — Upper Cuspid, Labial Surface.

126

ANATOMY.

Cervical
Ridge

is bounded by five margins — mesial, distal, cervical, mesial-incisive, and
distal-incisive. The mesial margin is rounded from labial to mesial, and
slightly convex from the cutting-edge to the cervical line. The distal
margin is also rounded from labial to distal, presents a greater convexity,
and is somewhat shorter from the cutting-edge to the cervical line than
the mesial margin. By a continuation and final union of these two
lateral margins the cervical margin of the surface is formed, while by
their union with the cutting-edges the mesial and distal angles of the
crown are established. The mesial-incisive margin is usually slightly

concave near its center, although in
some instances it is convex. The
distal incisive margin responds to the
same description, although the con-
cavity, when present, is nearest the
point of the cusp. From the summit
of the cusp these two margins slope
away to join the mesial and distal
angles, the distal incline being about
one-fourth longer than the mesial.
This surface is generally of greater

ginalRidg.B I Meao-

■ marginal longitudinal than transverse extent,

Ridge

its greatest mesiodistal diameter being
from angle to angle, or at a point
immediately above them. The sur-
face is convex in every direction, and
is marked by a central longitudinal
ridge, usually well defined — the labial
ridge. Beginning at the summit of the cusp, this ridge is more or less
contracted laterally, but as it passes over the surface in the direction of
the root it becomes broadened and flattened, and gradually disappears
in the cervical portion. Upon either side of this ridge are the labial
grooves, well defined at their beginning, but which gradually blend into
the surface of the crown as they pass rootward. In some instances these
grooves are so strongly defined as to form a decided ridge upon the mesial
and distal margins of the surface; — these are the labial marginal ridges.
The labial ridge and the two labial grooves mark the developmental
lines of the crown, the former resulting from the middle lobe, which
in this tooth is much the largest of the three, while the latter denotes
the line of junction between the middle and the lateral lobes.

The Lingual Surface of the Crown (Fig. 71).— This surface

Lidgual
Groove

Lingual
Groove

Fig

7i--

Lingual Ridge
Upper Cuspid, Lingual Surface.

UPPER CUSPID.

127

presents nearly the same general outlines as the labial, with the exception
of the cervical portion, which is more constricted, tending to produce an
oblong or egg-shape. It usually abounds in well-defined ridges and
depressions, giving to the tooth a rugged and strong appearance. There
is but little general concavity to the surface in passing from the point of
the cusp to the root, while it may be flat, concave, or convex. As in the
incisors, the margins of this surface are formed by three marginal ridges
and by the cutting-edge. The mesiomarginal ridge is commonly a well-
defined fold of enamel, beginning at the mesial angle and passing upward
in the direction of the root, where it
unites with the cervicomarginal ridge.
It is sometimes quite narrow and rather
sharply outlined; at others, it extends
well toward the center of the surface in
the form of a well-rounded fold. The
distomarginal ridge, which is some-
what shorter than the mesial, begins at
the distal angle and passes rootward to Ceovical
meet the cervical ridge. It is well Rid 8 e
rounded in every direction, but seldom
so well produced as the mesial. The
cervicomarginal ridge, which is formed
by a continuation or union of the two
former, nearly always partakes of their
nature, except when broken by the
presence of a cuspule, which is fre-
quently found upon this tooth (Fig. 72).

This small cusp of enamel may be bounded on one or both sides by a
fissure, which often extends well under the cervicomarginal ridge, and
sometimes completely separates it from the two lateral ridges. Passing
through the center of the surface from the summit of the cusp to the base
of the cervicomarginal ridge is the lingual ridge, which corresponds to
the labial ridge of the labial surface. This ridge is usually well pro-
duced at or near the point of the cusp, and may continue so throughout,
but most frequently becomes reduced in size near the center of the sur-
face. Between this ridge and the mesio- and disto-marginal ridges are
two longitudinal depressions — the lingual grooves.

Mesial Surface of the Crown (Fig. 72). — In general outline this
surface resembles that of the central incisor, excepting that the wedge-
shape which it describes is more heavily set and blunt, with the surface

Labial
Groovt

Cervico-
lingual
Ridge

Lingual
Ridge

Fig. 72.-

-Right Upper Cuspid, Mesial
Surface.

128

ANATOMY.

extending beyond the base of the cone, in the direction of the root, to
the extent of about one-third of its entire length. In some cases the base
of "the cone will be on a line with the cervical margin. The lower two-
thirds of the surface, or that nearest the mesial angle, is convex in every
direction; this convexity gradually disappears as the center is approached,
beyond which point it is much flattened, usually ending in a slight con-
cavity at the cervical margin. When looking directly upon this surface,
its margins will be found within the profile lines, these being represented
by the labial ridge anteriorly', and by the lingual and cervical ridges

posteriorly. The margins,
three in number, are the
labial, which is well rounded
and poorly defined; the //';/
gual, more or less distinctly
outlined and somewhat irreg-
ular; and the cervical, which
is represented by the extent of
the enamel covering of the
crown; this latter margin be-
ing concave in the direction of
the root. The most promi-
nent point of this surface
serves as a point of contact
for the distal surface of the
lateral incisor, the extent of
contact being much influenced
by the type of tooth, but in
the cuspid this is usually a
single point rather than an extent of surface.

The Distal Surface of the Crown (Fig. 73). — This surface in
many respects is similar to the mesial, particularly in its general outline.
The extent of surface is somewhat less and the convexity much more
marked than that of the mesial surface. The position of the distal
angle, which is the lower boundary of the surface, being much nearer
the cervical line, makes this surface about one-third shorter than the
mesial surface. The lateral margins of the surface, which are also within
the profile lines, differ from those of the mesial in being more clearly
defined. The cervical margin differs from that of the mesial surface
by having a concavity with much less depth. As stated above, the surface
is decidedly more convex than the mesial, the point of contact for the

Labial Sur-
face-

Summit of
Cusp.

Cervicolin-
gual Ridge

Distal
.Angle

Fig. 73.— Left Upper Cuspid, Distal

Surface.

UPPER CUSPID.

129

Cuspule
Lingual
Fossa

mesial surface of the first bicuspid being almost in the center. Near
the cervical margin the surface is inclined to flatness, and frequently
concave.

The Cutting-edge, or Cusp. — As inferred in the beginning of this
description, the cuspid tooth is both an incising and a penetrating organ,
the latter function being provided for by the presence of the single cusp,
which divides the cutting-edge into an anterior or mesial portion and a
posterior or distal portion. The mesial cutting-edge begins at the summit
of the cusp and slopes away to meet the mesial angle, which it assists
in forming. The outline of this edge is
usually gracefully curved and unbroken
unless permanently crossed by the labial
groove. The distal cutting-edge is gen-
erally somewhat longer than the mesial.
Immediately after leaving the summit of
the cusp it may be slightly concave; but
beyond this point it is well rounded,
until it reaches the distal angle,
which it gradually disappears. This
edge is also frequently broken by the
labial groove. In its entirety the cut-
ting-edge is subject to the same varia-
tions as those of the incisors — i. e., it may
be thin and sharp, or it may be thick
and blunt.

The Cusp. — The single cusp from which this tooth derives its name
is formed by the union of the labial ridge, the lingual ridge, and the
mesial and distal cutting-edges. The summit of the cusp is constant
in its position, always being in a direct line with the long axis of the
tooth, whether it be viewed from a mesial or a lingual direction.

The Cervical Line. — To describe this fully would be to repeat what
has already been said in connection with the incisor teeth. This enamel
margin differs in one particular only from that of the incisors, and that
variation is not a constant one — the lingual portion is frequently extended
in the direction of the root, producing a short, positive curve at that
point.

The Angles of the Crown. — Owing to the rounded, nature of the
majority of cuspid crowns, the term angle, as applied to its free extremities,
is almost a misnomer, and can only be considered as assisting in descrip-
tion. The mesial angle, which is formed by the union of the marginal

into Cervica
Ridgt

Lingual
Groovt

Marginal Ridges

Fig. 74. — Upper Cuspid, Lingual

Surface, Strongly Developed.

!3°

ANATOMY.

ridges of the labial and lingual surfaces with the mesial cutting-edge, is
seldom a well-produced angle, usually being rounded in every direction.
The distal angle, which is formed in a manner similar to the mesial, rs
somewhat more deserving of the name, both the labio- and linguo-margi-
nal ridges frequently presenting angularity. The position of the distal
angle is usually well toward the center of the crown, and occasionally
above this point, and, although it may descend, it is seldom found on
a line with the mesial angle.

The Neck of the Upper Cuspid. — This may or may not be a dis-
tinctive feature of the cuspid tooth, although when viewed from a labial
aspect, the lateral flare or bulging of the crown gives the appearance of a
decided constriction between the crown and the root; but, when examined
from the mesial surface, this constricted appearance is absent, the con-
tour of the crown passing into that of the root, with the cervical line alone
marking the extent of each. The tooth at this point is well rounded

anteriorly, flattened laterally, and again rounded
posteriorly, the latter forming the segment of a
-mailer circle than that of the labial surface.

The Root. — This tooth possesses the largest
and longest root of any of the teeth, in the latter
respect usually exceeding the central incisor by
about one-third, and the lateral incisor by one-
fourth or more. Like the base of the crown, it is
rounded on the labial and lingual surfaces, and is
flattened laterally, this form usually being continued
throughout its entire length. Itgradually diminishes
in size from the neck to the apex, and in its entirety
forms a perfect cone. On the mesial and distal
sides it is not only much flattened, but is frequently
provided with a longitudinal depression, which is
most marked near the center of its length. In
some instances this root is possessed of a slight distal
curve, which may be gradual from the base to the apex, or it may exist
in a more positive way by a sudden distal curve near its apical extremity.
Bilious Type (Fig. 75). — The rounded outlines common to the
cuspid tooth are less pronounced in this type than in any other, and
instead of curves, angles are present. The crown is above the average
size, length predominating over breadth, the cusp well formed, and the
angles strong. The labial surface is often crossed by a number of trans-
verse ridges near the cervical portion, the labial ridge is bold, as are also

Fig. 75. — Bilious Type.
Distal Surface.

UPPER CUSPID.

131

the labiomarginal ridges. The mesial and distal surfaces possess no
distinguishing features, but the lingual, like the labial, shows the angular
nature of the crown in having its margins and ridges squarely set.

A cuspule is more frequently found in this type than any other,
and sometimes reaches down to a point corresponding to the transverse
center of the crown. The neck is moderately well produced, and the
cervical line decidedly V-shaped on its lateral portions, while on the labial
and lingual it takes the form of a broken circle. The cutting-edges are
rather heavy and square, and are nearly of equal
length. In this temperament the cuspid tooth
often partakes of the form described in connection
with the lateral incisor of the same type — i.e., the
absence of the cutting-edge and one or both angles,
making the crown a perfect cone.

Nervous Type (Fig. 76). — The crown is of
much greater longitudinal than transverse extent,
the outlines oval and gracefully formed, and the
neck is much constricted from mesial to distal,
being made so by the lateral flare of the body of
the crown, which is a distinctive feature of this
type. The labial ridge is well formed near the
summit of the cusp, but usually disappears near
the center of the surface. The labiomarginal
ridges are seldom present, and the surface in
general is convex and smooth. The mesial and distal surfaces show a
pronounced convexity near the angles, and often a slight concavity
between this point and the cervical line. The lingual surface, while
generally showing all the descriptive lines, may be considered smooth;
it is convex from mesial to distal, and slightly concave in the direction
of the long axis of the tooth. The cusp is long and penetrating, the
distal cutting-edge is much longer than the mesial, and both are
inclined to sharpness. The cervical line on the labial and lingual
surfaces is deeply arched, frequently giving to the gingival margin a
receded appearance.

Sanguineous Type (Fig. 77). — The crown in this type abounds in
long curves, the longitudinal and transverse extents are nearly equal,
the angles, owing to their circular form, are barely deserving the name,
while the cusp and cutting-edges are outlined by one long, oval sweep.
The labial surface is prominent and convex, and the developmental
grooves are fairly well shown. The mesial and distal surfaces show a

Fig. 76. — Nervous
Type, Labial Surface.

132

ANATOMY.

moderate general convexity, while the lingual abounds in well-rounded
ridges and borders. The constriction forming the neck of the tooth
is moderate. The cervical line is in the form of perfectly arched curves,

forming on the labial surface the segment of
a circle corresponding to the circumference of
the crown of the tooth.

Lymphatic Type. — In this type the
crown is usually greater in its transverse than
in its longitudinal measurement; it is lacking
in graceful outline, and may best be described
as being short, thick, and heavy set. None
of the surfaces abound in descriptive lines,
although transverse ridges are sometimes
present on the labial surface near the cervix.
Both the labial and lingual surfaces are con-
vex in every direction, while the mesial and
distal are inclined to flatness. The cusp is
heavy and blunt, and the cutting-edges, which
are nearly of equal length, are thick and
dull. The mesial and distal angles are well
produced. The cervical line is almost a
direct line encircling the neck of the tooth,
the segmental form on the labial surface

Fig.

-Sanguineous

type, Distal Surfac e.

being that of a much larger circle than any of those previously described.
The neck is thick and heavy, and the roots generally short.

UPPER FIRST BICUSPID. 133

UPPER FIRST BICUSPID.

7th year 8th year 9th year 10th year nth year 12th year

FIG. 78.

Calcification Begins, from Four Centers, about the Fourth Year.
Calcification Completed, Eleventh to Twelfth Year.
Erupted, Tenth to Eleventh Year.
Average Length of Crown, .32.

Average Length of Root, .48.

Average Length over All, .80.

This tooth, although presenting a crown of vastly different contour,
is developed by a process almost identical with that of the incisors and
cuspids. As the name implies, it is made up of two cusps, one forming
the buccal and the other the lingual half of the crown. Calcification
in the buccal cusp is from three centers and begins about the fourth year,
the central lobe first receiving the lime salts. During the following
year the two lateral lobes begin to calcify, soon followed by a union of
the three, thus completing the margins and summit of the cusp. Unlike
the incisors, but corresponding to the cuspid, the middle lobe is much the
largest of the three, frequently forcing the developmental (buccal) grooves
well toward the angles of the crown. The development of the lingual
cusp corresponds to the development of the cervical ridge on the incisors
and cuspids, except that it has a separate center of calcification, and a
cusp almost as large as the buccal results. Between the fifth and sixth
year union between the two cusps takes place, the line of confluence
being permanently recorded by a well-defined groove, which traverses
the crown from mesial to distal. This groove, although forced to occupy
a different position, corresponds to the lingual groove of the incisors
and cuspids. After the union of the cusps the process of calcification
is continued into the body of the crown, and by the seventh year it is

134

ANATOMY.

Buccal Triangular Ridge

Mesial
Angle

Central
Groove

Buccal

Cusp

Buccal

Groove

Distal

Groove

Distomar-
ginal
Ridge

Lingual Cusp

more than half completed. The eighth year usually finds the crown
fully formed and the base of the root or roots outlined. As this tooth
is generally provided with two roots, the first indication of bifurcation
will be observed between the eighth and ninth year by a filling-in near
the center of the mesial and distal walls, which finally become united
by a thin septum of dentine or cementum. After this period the roots
calcify separately, and by the middle of the ninth year about one-third
of their length is established. During the following year, or at about
the time of eruption, the development of the roots has extended to about
three-fourths of their complete length, and between the eleventh and

twelfth year, or at a time corres-
ponding to that of the crown assum-
ing its final position in the arch, calci-
fication is completed (Fig. 78).

The Crown of the Upper First
Bicuspid presents for examination
five surfaces — buccal, lingual, mesial,
distal, and occlusal. In general, the
contour of the crown is irregularly
quadrilateral, being about one-third
greater in its buccolingual measure-
It is somewhat flattened from mesial
to distal, but rounded on its buccal and lingual surfaces.

The Occlusal Surface of the Crown (Fig. 79). — The contour
of the crown is best observed by a view of this surface, which may be
described as trapezoidal or irregularly quadrilateral in form. The
four margins of the surface are those which represent the four lateral
surfaces — the buccal, lingual, mesial, and distal, — the latter two being
in the form of well-defined ridges — the mesio- and disto-marginal ridges.
The buccal margin is formed by the mesial and distal inclines of the
buccal cusp; it has a slight buccal convexity, and at its union with the
proximate surfaces assists in forming the mesial and distal angles of the
crown. The distal half of this margin is usually somewhat longer than
the mesial, and the distal angle is less pronounced than the mesial. The
lingual margin presents a much greater convexity than the buccal, but
the curve formed is the segment of a much smaller circle. As in the
buccal margin, the distal half of this margin is the longest, but unlike
the former, its free extremities pass into the mesial and distal margins
without producing angles. The mesio- and disto-marginal ridges are
strong folds of enamel which arise from the mesial and distal angles and

Fig. 79. — Upper First Bicuspid,
Occlusal Surface.

ment than from mesial to distal.

UPPER FIRST BICUSPID. 1

converge slightly as they pass to the lingual, where they are gradually
lost in the lingual margin.

The Cusps (Fig. 79). — These are two in number, and are named,
in accordance with their location, buccal and lingual. The buccal cusp
is the larger and longer of the two. From the summit of this cusp four
ridges descend — one in a mesial direction, forming the mesial cutting-
edge of the crown; one in a distal direction, forming the distal cutting-
edge; one to the buccal surface, the buccal ridge; and a fourth, the buccal
triangular ridge, descends the central incline. The mesial and distal
ridges enter into the formation of the mesial and distal angles at their
extremities; the latter is slightly longer than the former, and both are
frequently broken near the center by the grooves of development — the
buccal grooves. The buccal ridge may be well developed and extend
almost to the cervical line, or it may be slight and disappear near the
center of the surface. The buccal triangular ridge usually ends some-
what abruptly in the central groove, but in some instances it is con-
tinued and joins a similar ridge from the lingual cusp, this union form-
ing the transverse ridge. The triangular ridge often bifurcates near
the center of its incline, and is continued in two distinct but smaller
ridges. The lingual cusp is much less angular than the buccal; the apex
is usually rounded, while the descending ridges are generally three in
number instead of four. The mesial and distal ridges are nearly of the
same length, and pass without interruption into the mesio- and disto-
marginal ridges. The triangular ridge is less clearly defined than its
fellow of the buccal cusp, and it is not unusual for it to be entirely
wanting. The lingual aspect of the cusp is smooth and rounded,
presenting nothing in the form of a ridge in correspondence with the
buccal ridge of the buccal cusp. Like the incisors and cuspids, the
summits of these cusps are usually in a direct line with the long axis
of the tooth.

The developmental grooves, all of which are observed upon the
occlusal surface, are the central, mesial, distal, two triangular, and two
buccal. The central groove is the most marked, is deeply sulcate, and
extends through the center of the surface from mesial to distal, ending
just within the two marginal ridges in two irregularly formed depressions
or pits — the mesial and distal pits. This groove marks the line of union
between the buccal and lingual lobes. The mesial and distal grooves
are not always well defined, but may usually be observed as fine lines
passing over the central portion of the mesio- and disto-marginal ridges.
The mesio- and disto-triangular grooves begin in the mesial and distal

136

ANATOMY.

Buccal
Ridge

Mesial
Angle

Distal
Angle

Distal Cut-
ting-edge

Buccal Grooves

Fig. 80. — Upper First Bicuspid,
Buccal Surface.

pits, and pass in the direction of the mesial and distal angles, where they
are either lost, or may be traced as slight depressions passing over the
buccal ridges near the angles, and they may further continue over the

buccal surface in the direction of the
root. These two grooves, together
with the mesial and distal above re-
ferred to, form the outlines of the
mesiobuccal and distobuccal develop-
mental lobes. The buccal grooves
will be described in connection with
the buccal surface. Supplemental
grooves are seldom found in connec-
tion with the buccal half of the oc-
clusal surface, but are occasionally
present on the central incline of the
lingual cusp.

The Buccal Surface of the
Crown (Fig. 80). — In many respects
this surface resembles the corresponding or labial surface of the cuspid
tooth. It is bounded by four margins — occlusal, mesial, distal, and
cervical. The occlusal half of the surface is formed of the buccal cusp,
and is cone-shaped, while the cervical
half is irregularly quadrilateral in form.
The extent of surface from the cervical
line to the point of the cusp is usually
about one-third greater than the greatest
mesiodistal diameter, which is repre-
sented by a line drawn from the mesial
to the distal angle. The form shown is
that of a general convexity, the summit
of which is surmounted by a longitudinal
ridge — the buccal ridge. This ridge,
which is formed from the central de-
velopmental lobe, is most pronounced
near the occlusal margin, and gradually
disappears near the center of the surface.
Upon either side of the buccal ridge are
two grooves — the buccal grooves — which denote the line of union between
the central and the two lateral lobes, and beyond these are the angles of
the crown. The buccal ridge springs from the buccal cusp, the summit of

Lingual
Groove

Summit of Lingual Cusp

Fig. 81. — Upper First Bicuspid,

Lingual Surface.

UPPER FIRST BICUSPID.

1 31

which is generally in a direct line with the long axis of the tooth; when
there is a deviation from this the summit is usually thrown a little to
the mesial, resulting in a reduction of the length of the mesial cutting-edge.

As previously stated, the greatest mesiodistal diameter of the surface
is on a line with the angles of the crown; this measurement is much reduced
at the cervical line, so that the point of contact with adjoining teeth is
thrown near the occlusal margins of the crown. This variation in the
transverse measurement also results in what is commonly referred to as
the "bell shape" of the crown. The cervical margin of the surface is
fairly well arched, but seldom to such a degree as the corresponding
margin upon the incisors and cuspids.

The Lingual Surface of the Crown (Fig. 81). — This surface is
smooth and decidedly convex, the absence of strongly developed grooves
and ridges contributing to the former fact. Like the buccal surface,
its greatest transverse measure-
ment is at the base of the cusp in
which it terminates. The extent
of the surface is about one-third
less than that of the buccal, and
its occluding and cervical margins
are alone well defined, the mesio-
distal convexity passing so gradu-
ally into these respective surfaces
that a positive line of distinction Mesiomar-

can scarcely be recognized. In a ' ' . - "" "~~ L cu^p al

passing from the cervical line to
the occlusal margin, the surface is Fig. 82.-

rapidly carried toward the center
of the crown. The cervical margin of this surface is usually in the form
of a direct line encircling the neck, but occasionally presents a slight
concavity in the direction of the root.

The Mesial Surface of the Crown (Fig. 82). — This surface of the
crown has three of its borders well defined; these are the buccal, the
occlusal, and the cervical, the remaining or lingual margin passing so
gradually into the lingual surface that no positive line of demarcation
can be given. The buccal margin extends from the mesial angle to the
cervical line, and invariably presents a slight buccal inclination, thus
increasing the width of the surface on its cervical portion. The occlusal
margin is formed by the mesiomarginal ridge, and by a portion of the
ridge descending from the lingual cusp. It is irregularly V-shaped,

Cervical
Line

Buccal
Ridge

-Upper First Bicuspid, Mesial
Surface.

ANATOMY.

Lingual
Root

Buccal Root

Distomar-
ginal Ridge
Lingual
Cusp

Buccal
Ridge

and in many instances is broken in the center by the mesial groove. The
cervical margin differs from those of the incisors and cuspids, nearly
always being in the form of a straight line from buccal to lingual. The
surface in general is flattened, but shows a slight general convexity near
the occlusal margin, and frequently a slight concavity immediately below
the cervical line, this form placing the point of contact with the distal
surface of the cuspid near the occlusal margin. The surface is occa-
sionally divided into a buccal and a lingual portion by the mesial groove.
which may extend to the cervical line, but which generally disappears

near the center of the surface.
The buccal half of the surface
which is formed from the mesial
developmental lobe is inclined to
angularity, while the lingual half
is decidedly rounded, particularly
in the direction of the occlusal
margin.

The Distal Surface of the
Crown (Fig. 83). — In general,
this surface resembles the mesial,
being flattened and bounded by
three more or less distinct mar-
gins. The slight buccolingual
convexity is not confined to the
occlusal portion of the surface,
but is inclined to extend to the
cervical margin, in this particular being at variance with the mesial
surface. This surface passes into the lingual by a much longer curve
than that shown on the mesial surface.

The Angles of the Crown. — These are two in number and, as in
the teeth previously described, are named, according to their location.
mesial and distal. The mesial angle is formed by the union of the mesio-
marginal ridge and mesial cutting-edge. It is primarily the product of
the mesial developmental lobe, and is usually well produced. The
distal angle, which is formed in a like manner, is inclined to be more
rounded.

The Neck of the Tooth. — In most typal forms the neck of the
upper first bicuspid is well defined, particularly upon the mesial and
distal surfaces. Viewing the tooth from a buccal aspect, the neck is
a distinctive feature, but when studied from the mesial or distal sides,

Triangular Ridge,
Occlusal Surface

Fig. 83. — Upper First Bicuspid, Distal

Surface.

UPPER FIRST BICUSPID. I39

the constriction is scarcely observed, this being particularly the case if
the tooth has but a single root. In general, the neck partakes of the
contour of the crown, being convex on the buccal and lingual, and flat-
tened and frequently slightly concave on the mesial and distal.

The Roots of the Upper First Bicuspid. — This tooth is usually
developed with two roots, sometimes with only one, and in rare instances
it may have three. When two roots are present, one is above the buccal
and the other above the lingual half of the crown, and are named, accord-

FiG. 84. — Types of Bicuspids.

ing to their location, as buccal and Ungual. In general form the two
roots are quite similar, but the buccal is usually a trifle longer than the
lingual. They taper off to a slender apex, and are inclined to curve in
various directions near their extremities. The point of bifurcation is
frequently some distance above the neck, so that the tooth may be said
to possess a single root with two branches. Below the bifurcation the
root assumes the form of the neck or cervical portion of the crown, but
as the bifurcation is approached, the mesial and distal sides present a
longitudinal groove, which gradually increases in depth until the single
root becomes separated. The curves of the root-branches above referred
to are, in the buccal branch, first to the buccal and then to the lingual;
while the lingual branch first shows a slight lingual inclination immedi-
ately above the point of separation, followed by a gentle buccal curve
as the apical end is reached. In some cases the bifurcation begins

140

ANATOMY.

immediately above the neck of the tooth; in others it may occur in the
apical third; while a third class is represented by the two roots being
united throughout their entire length by a thin septum of dentin and
cementum, or, as occasionally happens, by a layer of cementum alone.
In this latter instance each root is provided with a distinct canal and
foramen. When the tooth has but a single root, it is much flattened
from mesial to distal, the flatness being slightly broken by an inclination
to convexity. The four surfaces usually converge toward the apical end,
which is oblong from buccal to lingual, and generally provided with a

slight distal curve. The presence, of
three roots is so rare that the condi-
tion might be classed as a malforma-
tion; but when they do exist, two are
usually attached to the buccal and one
to the lingual half of the crown, with
the point of separation near the neck
of the tooth.

Bilious Type (Fig. 85).— The
upper first bicuspid of this tempera-
mental type is marked by a crown of
moderate length, the neck well pro-
nounced, and the cusps and angles
marked by angular outlines. The
buccal ridge is strongly defined, and
the buccal grooves, which extend well up on the buccal surface, cross the
mesial and distal cutting-edges, separating them into two distinct parts.
The cusps are long and penetrating, and are nearly of equal length,
assisting to form the firm and well-locked occlusion common to this type.
The mesial and distal surfaces are nearly parallel with each other; they
are seldom convex, so that the approximating teeth are in contact over
an extent of surface rather than a single point. The cervical line is but
little curved.

Nervous Type (Fig. 86). — In this temperament the bell-shaped
crown is strongly observed, the crown being long and much con-
stricted at its neck. The extreme length of the buccal cusp, and the
marked cervical constriction, produce an appearance in the buccal
surface resembling the labial surface of the cuspid tooth. The develop-
mental grooves are finely outlined, and the cusps long and penetrating,
usually being more pronounced than in any other class. The cutting-
edges are sharp and inclined to angularity; the mesial and distal surfaces

Fig. 85.— Bilious Type, Distal
Surface.

UPPER FIRST BICUSPID.

141

are convex near their occlusal margins, but near the cervical line a
pronounced concavity is observed, which is continued upon the correspond-
ing root-surfaces. This formation forces the point of contact with
adjoining teeth well toward the occlusal surface, and results in an exten-
sive V-shaped interproximate space. The cervical line is sharply and
gracefully formed, the curvature being well arched.

Sanguineous Type. — The typical upper first bicuspid of this class
is provided with a crown well proportioned, its length being somewhat
greater than its breadth, but about equal to its buccolingual measure-
ment. The buccal surface is seldom broken by the
buccal grooves, and is strongly convex in every direc-
tion. The lingual surface is much more rounded
than the same surface of other types. The mesial
and distal surfaces are usually smoothly convex, with
an occasional slight concavity immediately below the
cervical line. Upon the occlusal surface the grooves
are rounded and obscure, rather than sharp and well
defined, and the cusps, much less pronounced than in
either of the types previously described, are rounded
and smooth; this latter fact is particularly true of the
lingual cusp, which is usually much smaller than the
buccal. The cutting-edges of the buccal cusp are
scarcely deserving of the name, being broad and
rounded throughout. The form of the mesial and
distal surfaces above described provides for a point of contact near the
center of each surface, leaving a slight interproximate space both above
and below this point.

Lymphatic Type. — An examination of the upper first bicuspid of
the lymphatic type results in finding a tooth vastly different from any
of those previously described. The length of the crown from the cervical
line to the point of the cusp is less than either the mesiodistal or bucco-
lingual measurements. In general appearance it is lacking in symmetry,
or poorly proportioned. The buccal surface presents a gradual convexity
from mesial to distal, and seldom has the buccal ridge well developed.
The lingual surface is smoothly convex and passes off into the lingual
root without the interposition of a decided neck. The mesial and distal
surfaces are flattened and sparingly convex, and are nearly parallel with
each other, so that the contact with adjoining teeth is inclined to be
distributed over the entire surface, leaving little or no interproximate
space. The cusps are short, flat, and rounded, and the occlusal surface

Fig. Ho. — iNerv-
ous Type, Buccal
Surface.

142 ANATOMY.

much flattened in general, corresponding with the nature of the occlusion,
which is loose and wandering. The developmental grooves and ridges
are fairly well shown, while the cutting-edges and angles of the crown
are smooth and rounded. The neck is less pronounced in this type than
in any other, the curvature of the cervical line is very slight, the root is
short and heavy set, frequently passing well up toward the apex before
bifurcating.

UPPER SECOND BICUSPID. 143

UPPER SECOND BICUSPID.

7th year 8th year 9th year 10th year nth year 12th year

Fig. 87.

Calcification Begins, from Four Centers, about the Fifth Year.
Calcification Completed, Eleventh to Twelfth Year.
Erupted, Eleventh to Twelfth Year.
Average Length of Crown, .29.

Average Length of Root, 55.

Average Length over All, .84.

The process of development in this tooth is identical with that of
the first bicuspid, calcification in the buccal half of the crown taking
place in one central and two lateral lobes, while the lingual half is developed
from a single center. The calcifying process is about one year later
than that in the first bicuspid, the summit of the buccal cusp receiving
its lime salts about the beginning of the fifth year. During the following
six months calcification begins in the lateral lobes, and also in the lingual
lobe. By the sixth year the occlusal surface and a portion of the crown
are completed by a union of the various lobes, and at seven years the
crown is calcified for more than two-thirds of its completed length.
Between the eighth and ninth year the contour of the crown is established,
and the neck of the tooth and outline of the root-base formed. At the
tenth year about one-third of the root-length is formed, and during the
following year about 1/8 of an inch is added to it. By the eleventh or
twelfth year calcification is completed (Fig. 87).

This tooth so closely resembles the first bicuspid that a description
in detail will be unnecessary; there are, however, a few minor points
which are at variance and must be described in order to distinguish one
from the other. In general, the tooth is a trifle smaller than the first
bicuspid, the cusps are somewhat shorter, and the various ridges less

144

ANATOMY.

Central
Groove

Mesiomar-
ginal Ridge

Fig. 88. — Second Upper Bicuspid
Occlusal Surf;u e.

distinct. A distinguishing feature of the occlusal surface is found in the
diminished length of the central groove (Fig. 88). This groove, as
observed in the first bicuspid, extends from mesial to distal for fully
three-fourths of the entire width of the surface; but in the second bicuspid
it is diminished by one-third, being thus reduced by the broadened

marginal ridges, which force the mesial and
distal pits well toward the center. It is
not uncommon to find the triangular grooves
joining the central groove directly in the
Triangular center of the surface, forming a central pit

Grooves

from which may radiate numerous small
supplemental grooves and ridges. The
summits of both the buccal and lingual
cusps are nearer to the mesial than to the
distal surface, thus increasing the length of
the ridges which descend from them in a distal direction, and decreasing
those which pass to the mesial. The buccal surface presents a greater
convexity than that of the first bicuspid; the buccal grooves are usually
shallow depressions and are frequently entirely wanting, thus giving the
buccal ridge the appearance of extending its margins to the angles of
the crown. Unlike the first bicuspid,
the mesiodistal diameter of the mesial
surface is but little more than the
same measurements on the lingual
surface. The neck of the tooth is
not quite so pronounced as that of
the first bicuspid, thus giving less of
the bell shape to the crown. One
very important difference between
this tooth and the first bicuspid is in
the root-formation. We have seen
that the first bicuspid is generally pro-
vided with two roots, while in the
second bicuspid a single root is
usually present. Like the first bi-
cuspid, there are exceptions to this,

the tooth sometimes being provided with two, and in rare instances with
three, roots. When the single root is present, it partakes of the form
of the crown at its base, being well rounded on the buccal and lingual
portions and much flattened on the mesial and distal. The mesiodistal

Cervical
Line

Buccal

1 ■

Ridge

Mesiomar-

ginal Ridge

Buccal
Surface

Lingual
Cusp

Fig 89. — Upper Second Bicuspid, Me-
sial Surface. Most common form.

UPPER SECOND BICUSPID.

145

diameter of the root at its base is only about one-third that of the bucco-
lingual measurement, this proportionate size continuing throughout
its entire length. In passing from the base to the apex the root is gradu-
ally diminished in size, finally ending somewhat

abruptly in an oblong extremity. In some in-
stances the apical end is round and pointed,
resembling the apex of the incisors and cuspids,
but when thus formed the root is usually curved
near its apical third and somewhat extended in
length. The mesial and distal sufaces are pro-
vided with a well-defined longitudinal concavity,
extending from the cervical margin to the apex and
dividing the root into a buccal and a lingual por-
tion. This depression is often so decided that the
contour of the single root is almost lost, and in its
place the appearance is that of two roots similar to
those described in the first bicuspid. The length
of the root is usually a little greater than that of
the first bicuspid, but the crown being a trifle
shorter, results in producing a tooth the entire
about equal to that of the first bicuspid.

Fig. 90. — Young Up-
per Second Bicuspid,
Lingual Surface.

length

of which is

146

ANATOMY.

UPPER FIRST MOLAR.

g>~& ft V f

2d year 3d year 5th year Gth year 7 th year

Fig. 91.

10th year

1 1 th vear

Calcification Begins, from Four Centers, about Onk Month Before Birth
Calcification Completed, Ninth to Tenth Year.
Erupted, Sixth to Seventh Year.
Average Length of Crown, .30.

Average Length of Root, .51

Average Length over All, 81

This tooth being the first of the permanent organs to erupt, it
precedes all others in the process of calcification, beginning to receive
its lime salts as early as the eighth fetal month. The form of the crown
being so entirely different from those previously described, embodies
a developmental process which is also different, four distinct lobes being
present, one for each cusp, these making their appearance during the
first year after birth, closely followed by a completion of the occlusal
surface by the union of the free calcifying margins, these lines of union
being finally represented by the developmental grooves of the occlusal
surface. After the completion of this surface, calcification proceeds in
the direction of the base of the crown, and at the beginning of the third
year about two-thirds of the crown is formed. During the fifth year
(he contour of the crown is completed, and at the beginning of the eruptive
period, or about the sixth year, the developmental process has extended
to the base of the roots, and an effort at trifurcation begun. At seven
years the three roots with which the tooth is provided are branching out,
each into its own socket, subsequent development in each being con-
tinued as a separate and distinct process. The tenth year more than
half completes the calcifying process in the roots, and at the beginning
of the eleventh year the root apices are formed. Like the first bicuspid,
the crown of this tooth presents for examination five surfaces — occlusal,
buccal, lingual, mesial, and distal. In general contour it i^ irregularly

UPPER FIRST MOLAR. 147

quadrilateral, with the angles of the crown more or less rounded, two
of its sides convex and two flattened or slightly concave. The length
of the crown from the cervical line to the summits of the cusps is about
equal to, or slightly less than, its mesiodistal diameter, while the bucco-
lingual measurement is usually a trifle greater than the mesiodistal.

The Occlusal Surface of the Crown (Fig. 92). — The coronal
outline of this tooth is best studied when looking directly upon this
surface, which shows the two convex sides above referred to, represented

Mesiobuccal Cusp

Buccal Groove

Central Fossa B ■ Dist ° buccal

^^ ^K Cusp

Oblique Ridge ^" f 1 I Distal Fossa

Mesiolingual Cusp W^. ^T 'B Dist P5' a i'"

The Fifth Cusp ^^^ ^^

Distolingual Distolingual
Groove Cusp

Fig. 92. — Upper First Molar, Occlusal Surface.

by the buccal and lingual margins, with the mesial and distal margins
more or less flattened. The surface is bounded by these four margins,
which are nearly of equal length, the angles formed by their union being
more or less rounded, two of which, the mesiobuccal and the distolingual,
are acute angles, while the mesiolingual and distobuccal are obtuse angles.
The surface is divided into four developmental portions — the mesiobuccal,
distobuccal, mesiolingual, and distolingual. Each one of these parts
is surmounted by a well-defined point or cusp, which likewise is named
in accordance with its location. These various parts are separated
from one another by four developmental grooves — the mesial, the buccal,
the distal, and the distolingual. In the center of the triangle formed
by the central incline of the mesiobuccal, distobuccal, and mesiolingual
cusps is a deep depression — the central fossa — while near the distal
margin is a somewhat similar depression — the distal fossa. Traversing
the surface in various directions are a number of ridges and supplemental
grooves, each of which will be described in turn.

The Marginal Ridges of the Occlusal Surface. — These are four in
number — the mesial, distal, buccal, and lingual. The mesiomarginal
ridg£ is a well-prortounced elevation of enamel which passes from the
mesiobuccal angle to the mesiolingual angle. It is slightly concave in

148 ANATOMY.

the direction of the root, and is broken near the center of its concavity
by the mesial groove, upon either side of which are frequently found one
or two small points or tubercles, which are formed either by a division
of the mesial developmental groove, or by one or more supplemental
grooves. These grooves pass over the ridge and are continued for a short
distance on the mesial surface. Descending from the mesiobuccal cusp,
the ridge passes in a lingual direction to meet the mesiolingual cusp, and
in so doing has a slight distal inclination until the mesial groove is reached,
after passing which it makes a sweeping distal curve and is lost in the
lingual margin. This ridge marks the line of junction between the
occlusal surface and the mesial surface. The distomarginal ridge in
some respects resembles the mesial just described, being concave and
ascending in a buccal and lingual direction, with a somewhat rounded
outline, to the summits of the distobuccal and distolingual cusps. The
depth of the concavity is usually greater than that of the mesial margin,
and is frequently crossed near the center by the distolingual groove,
frequently so marked as to produce a V-shape to the center of the margin.
There are occasionally found upon either side of this central groove one
or more small tubercles, corresponding to those of the mesial ridge, but
they are less frequent and less pronounced. This ridge forms the line
of demarcation between the occlusal surface and the distal surface. The
buccomarginal ridge begins at the mesiobuccal angle, and gradually
ascends to the summit of the mesiobuccal cusp, from which it afterward
descends in a distal direction to the buccal groove; continuing, it again
ascends the distobuccal cusp, after descending from which it ends in the
distobuccal angle. The nature of this ridge is a series of cutting-edges,
giving to the cusps their angular nature. Besides the buccal groove,
which makes a decided break in the center of its course, the ridge is
frequently crossed by numerous small supplemental grooves occurring
in various locations and forming a series of minute tubercles; this latter
condition is most frequently present in young teeth, and is soon obliterated
by wear. The course of this ridge is not that of a direct line from mesial
to distal, but in its ascent of the mesiobuccal cusp it is inclined to the
buccal; in its descent it presents a corresponding return to the lingual,
and the same variations are observed in passing over the distal cusp.
The linguo marginal ridge begins at the mesiolingual angle of the crown
and passes distally to the distolingual angle, differing from the three
previously described by being heavy and rounded in its nature, more
irregular in outline, and divided nearest to its distal extremity instead
of in the center of its length. From the point of beginning it makes a

UPPER FIRST MOLAR. 149

curved ascent to the summit of the mesiolingual cusp; descending from
this in a distobuccal direction, it divides, one portion passing to join the
triangular ridge of the distobuccal cusp, the two uniting to form the
oblique ridge, the other portion continuing in the direction of the disto-
lingual cusp, before reaching the base of which it is broken by the disto-
lingual groove. From this groove the ridge makes a sudden and direct
ascent to the summit of the distolingual cusp, after passing which it
gradually descends in a long curve to join the distomarginal ridge. Like
the buccal ridge, it is frequently crossed by numerous supplemental
grooves. The ridge forms the lingual margin of the occlusal surface,
and gives to the cusps their angularity.

The Cusps (Fig. 92). — These are four in number — the mesiobuccal,
distobuccal, mesiolingual, and distolingual.

The Mesiobuccal Cusp (Fig. 92). — In extent of surface this is usually
the largest cusp, although it is sometimes. exceeded by the mesiolingual.
From the summit of the cusp three ridges descend — the buccal ridge to
the buccal surface, the buccomarginal ridge making a double descent,
and the mesiobuccal triangular ridge, the latter descending the central
incline and ending in the central fossa. The mesial base of the cusp is
frequently crossed by one or more supplemental grooves, which begin at
the mesial margin and pass in the direction of the central fossa. The
central slope of the cusp contributes to the formation of the central fossa,
its extent in this direction being controlled by the mesial and buccal
grooves, which together form the mesiobuccal triangular groove.

The Distobuccal Cusp (Fig. 92). — This cusp is frequently the smallest
in extent of surface, but is usually longer and more pointed than the others.
Like the mesiobuccal cusp, three ridges descend from it — the buccal
ridge to the buccal surface two which spring from the buccomarginal
ridge, and the distobuccal triangular ridge, which descends obliquely
toward the distal center of the surface and joins a similar ridge (pre-
viously described) from the mesiolingual cusp, the two forming the
oblique ridge. The mesial portion of the base of this cusp assists in
forming the central fossa, while a portion of the distal contributes to the
formation of the distal fossa. The inner boundary of the cusp is formed
by the buccal groove, the distal groove, and by a portion of the disto-
lingual groove.

The Mesiolingual Cusp (Fig. 92). — As above stated, this cusp is
frequently the largest in extent of surface, and is somewhat rounded,
with its summit poorly defined. The ridges which descend from it
correspond in name and number to those of the buccal cusps, the linguo-

150 ANATOMY.

marginal ridge making a double descent, the mesiolingual ridge descend-
ing to the lingual surface, while the central incline is marked by the
mesiolingual triangular ridge, which ends in the central fossa. Toward
the mesial portion of the cusp one or more small ridges are frequently
present, extending from the marginal ridge to the mesial groove. The
distal descent of the marginal ridge is bifurcated, one portion making
a sweeping curve and joining the transverse ridge from the buccal cusp,
forming the oblique ridge previously referred to, the other portion
passing in a distal direction and ending at the distolingual groove. The
central incline of this cusp forms the lingual side of the central fossa,
and its boundaries are outlined by the mesial, distal, and distolingual
grooves.

The Distolingual Cusp (Fig. 92). — This cusp is usually the smallest
of the four; it is triangular in outline, with the summit nearest the mesio-
lingual portion. The ridges which descend from this cusp are only two
in number, one passing in a mesial direction and forming a portion of the
linguomarginal ridge, the other passing to the distal, with a gradual
buccal curve, to join the distomarginal ridge. Of the two remaining
inclines, one looks in a distolingual direction, presenting a surface which
is smooth and rounded; the other, sloping by a broad expanse in a mesio-
buccal direction, ending in the distolingual groove, and also assisting to
form the distal fossa. This latter incline is often crossed by small sup-
plemental grooves, which take a winding course from the base to the
summit of the incline. The inner margin or outline of the cusp is formed
by the distolingual groove.

The Fifth Cusp (Fig. 92). — Although usually referred to as possessing
but four cusps, this tooth is frequently developed with five, the additional
lobe being situated on the lingual side of the mesiolingual cusp, about
midway between its summit and the neck of the tooth. When present,
it is distinctly separate from the main cusp by a well-developed groove —
the mesiolingual groove. Both the cusp and the groove may be more or
less developed, the former in some instances assuming dimensions cor-
responding to that of the distolingual cusp, and the latter sometimes
being as well marked as the distolingual groove. When thus pronounced,
the groove begins near the center of the mesial surface, and passes ob-
liquely toward the summit of the mesiolingual cusp, before reaching which
it makes an abrupt turn rootward, and joins the lingual terminal of the
distolingual groove, this union frequently resulting in a well-defined pit—
the lingual pit. This cusp, as usually found, is small and apparently
without function. When occurring on the tooth of one side, it is usually

UPPER FIRST MOLAR.

present on the corresponding tooth of the opposite side. It is seldom
present on any but the upper first molar.

The Fossae and Grooves of the Occlusal Surface (Figs. 92 and
93). — The fossae are two in number — central and distal. The central
jossa occupies a position near the center of the surface, and is formed by
the central incline of the mesiobuccal, distobuccal, and mesiolingual
cusp, which usually give it a three-sided form. Connecting the three
sides of the fossa, and in a measure assisting in its construction, is the

Buccal Groove

Distobuccal
Cusp

Distal Groove

Distolingual
Cusp

Mesiobuccal

Cusp

Central Fossa

Mesiomar-
ginal Ridge

The Fifth
Cusp

Mesiolingual Oblique
Cusp Ridge

Fig. 93. — Upper First Molar, Occlusal Surface, Strongly Developed, showing

Presence of Fifth Cusp.

mesiomarginal ridge and the oblique ridge. The depth of this fossa,
as well as that of the distal, is of course regulated by the length of the
cusps, which in turn is much influenced by the temperamental type of the
tooth. The bottom of the fossa is deeply marked by two of the grooves
of development — the mesial groove and the buccal groove. The former
begins on the mesial surface, passes over the mesiomarginal ridge, and
continues in an irregular line to the bottom of the fossa; the latter, begin-
ning near the center of the buccal surface, enters the fossa by crossing
the buccomarginal ridge near the center of its length, and also ends in
the central pit of the central fossa. As previously referred to, the union
of these two grooves forms the mesiobuccal triangular groove. From
the central pit of this fossa another groove is given off — the distal groove.
It is usually well defined at its beginning, but as it passes over the oblique
ridge it is generally partly obliterated, although occasionally being so
marked as to divide this ridge. The distal fossa is much smaller than
the central, and is of an entirely different form. Its walls are principally
formed by the distolingual incline of the oblique ridge, and the mesio-
buccal incline of the distolingual cusp; a portion of the distomarginal

152

ANATOMY.

Distobuccal
Root

Mesiobuccal
Root

ridge and the distal incline of the distobuccal cusp also assist in its forma-
tion. Like the central fossa, its sides are more or less irregular, from
the presence of various grooves and ridges in its vicinity. The greatest
length of the fossa is in a distolingual direction, and it is traversed by
a deep developmental groove — the distolingual groove. When the distal
groove crosses the oblique ridge, it usually extends to the bottom of this
fossa.

The Buccal Surface of the Crown (Fig. 94). — This surface, which
is the result of a union between the mesial and distal developmental

lobes, may be divided into a
mesial and a distal half. These
two portions are quite similar
in outline, and are separated
J Lingual Root from each other by the buccal
groove, which usually ends near
the center of about half-way to
the cervical line in a decided
pit — the buccal pit. In some
instances this groove is con-
tinued to the cervical line, or
even beyond this to the bifur-
cation of the roots. Both the
mesial and distal half are pro-
vided with a longitudinal ridge
(the buccal ridges) — one the
mesiobuccal ridge and the other
the distobuccal ridge. These are
similarly formed and descend from the summits of the respective cusps,
at which point they are usually well denned, but gradually disappear as
they pass toward the cervical line. The location of the buccal groove being
a little to the distal of the center of the surface, gives to the mesial portion
a somewhat greater extent than the distal. The margins of the surface,
which form an irregular quadrilateral, are the mesial, distal, occlusal, and
cervical. The mesial and distal margins are rounded, and gradually con-
verge as they pass rootward, making the average diameter of the surface
about one-fourth less at the cervical line than at the base of the cusps.
In some instances these margins are slightly concave over their cervical
portion, and convex on approaching the occlusal margins; or the mesial
may be concave and the distal convex throughout their entire length;
in some types they appear as straight lines and are parallel with each

Distobuccal
Cusp

FlG. 94.

Mesiobuccal
Cusp

Buccal Groove

-Upper First Molar, Buccal
Surface.

UPPER FIRST MOLAR.

J 53

Distobuccal
Root

Mesiobuc-
cal Root

other. The occlusal margin is formed by the marginal ridges as they
pass over the two buccal cusps, being in the form of the letter W. The
cervical margin is usually a direct line drawn around the circumference
of the tooth, but in some instances deviating slightly from this. Immedi-
ately below the cervical line, and conforming to its general direction,
is a rounded fold of enamel — the cervical ridge.

Lingual Surface of the Crown* (Fig. 95). — In correspondence
with the buccal surface, this surface is developed from two lobes —
the -mesio- and disto-lingual lobes — the line of union between the two
being recorded by a well-
defined groove — the lingual
groove. This groove, which
is a continuation of the dis-
tolingual groove of the occlusal
surface, usually ends near the
center of the lingual surface in
a well-defined pit — the lingual
pit — or it may continue root-
ward and gradually disappear.
It is located a little to the
distal of the center of the sur-
face, thus making the mesial
a trifle larger than the distal
portion. The mesial half of
the surface is smooth and

convex; the lingual incline of the mesiolingual cusp is seldom provided
with a well-defined ridge, although usually referred to as the mesio-
lingual ridge. The distal half of the surface is also smooth and
rounded, with the mesiodistal convexity much more marked than that
of the mesial lobe. The cervical ridge is seldom so pronounced as
that of the buccal surface, but the enamel frequently makes a
sudden dip at this point to meet the cementum of the root. That
portion of the surface immediately below the cervical ridge is smooth
and unbroken, slightly convex in the direction of the long axis of the
tooth, and flattened or slightly convex from mesial to distal. The
margins of the surface are the mesial, distal, occlusal, and cervical.
The surface passes so gradually into the mesial and distal that it is
somewhat difficult to define these margins. In general, the margins

Distolingual
Angle

Fig. 95.

Lingual
Groove

Lingual Ridge
Mesiolingual Cusp

-Upper First Molar, Lingual
Surface.

*When the fifth cusp is present, the anatomy of the mesial half of this surface is some-
what more complex.

154

ANATOMY.

Lingual
Root

Cervical
Line

Mesiolingual
Angle

Mesiobuc-
cal Root

converge slightly in the direction of the root. Both the occlusal and
cervical margins are similar to the corresponding margins of the buccal
surface.

The Mesial Surface of the Crown (Fig. 96). — This surface is
almost an unbroken plane, being smooth and flat. In some instances
it is crossed near the center of its occlusal margin by a continuation of
the mesial groove, but this is seldom so pronounced as to divide the
surface. The occlusal third of the surface is inclined to a slight general
convexity, providing a point of contact for the distal surface of the second

bicuspid, but between this
and the cervical line there is
often a slight concavity. The
margins of the surface are the
occlusal, buccal, lingual, and
cervical. The first named is
formed by the mesiomarginal
ridge of the occlusal surface,
and is concave in the direc-
tion of the root. The buccal
and lingual margiris are
rounded, and, unlike the
lateral margins of the buccal
and lingual surfaces, diverge
in the direction of the roots. The cervical margin is slightly concave in
the direction of the occlusal surface, and its length is much greater than
that of any other margin of the crown. This surface is more extensive
than either the buccal, lingual, or distal, and is about equal to that of
the occlusal. When the fifth cusp is present it alters the form of the
lingual margin of this surface by crossing it near the center, and ex-
tending for some little distance on the face of the surface.

The Distal Surface of the Crown (Fig. 97). — Taken in its entirety,
this surface usually presents a general convexity. The lingual half of
the surface is usually somewhat more prominent than the buccal, the
latter being flattened and frequently slightly concave, particularly near
the cervical portion. In some instances the surface is traversed by a
continuation of the distolingual groove, which, after passing over the
distomarginal ridge, is continued in a longitudinal direction, dividing
the surface into two equal parts. Not infrequently this groove, instead
of existing as such, is represented as a shallow depression, often extend-
ing to the bifurcation of the roots. The margins of the surface are four

Mesiobuc-
cal Angle

Fig. 96. — Upper First Molar, Mesial
Surface.

UPPER FIRST MOLAR.

r 55

Lingual
Root

Mesiobuc-
cal Root

Distobuc-
cal Root

Disto-

lingual

Cusp

Distobuc-
cal Cusp

in number: the occlusal, which closely resembles the corresponding
margin of the mesial surface; the buccal, which is not well defined; the
lingual, somewhat angular; and the cervical, formed by the cervical line.

The Neck of the Tooth. — When looking upon the buccal surface
of this tooth, the constricted portion forming the neck is greatest at a
point immediately above the cervical line. Viewed in this direction the
crown is usually bell-shaped, and both the crown and the base of the
roots assist in producing the neck. Viewed from a lingual direction,
the neck is a distinctive feature, but is seldom so marked as when ex-
amined from the opposite side.
When studied from either a mesial
or a distal aspect, the neck appears
above the cervical line, the promi-
nent fold of enamel immediately
adjacent to this line forcing the
neck rootward.

The Roots of the Upper First
Molar. — The roots of this tooth are
three in number, two of which are
on the buccal side, and are, there-
fore, called mesiobuccal and disto-
buccal roots, and one on the lingual
side, known as the lingual root.
These three roots are given off
from a common base, which is

sometimes referred to as the root, while those parts above the point of
trifurcation are considered as root-branches. The number, location,
and form of the roots of this tooth are, perhaps, more constant than those
found in connection with any other cuspidate tooth. The common base
from which the roots are given off is similar in contour to the crown of
the tooth, excepting in those cases in which the form of the root is carried
over this base to meet the neck of the tooth.

The mesiobuccal root (Fig. 97) is flattened from mesial to distal, broad
at its base from buccal to lingual, from which point it gradually tapers
to the apex. At the base the mesiodistal measurement is less than one-
third that of the buccolingual. In its course it is first inclined to the
mesial, but after reaching the center of its length it makes a decided
distal curve, which looks almost directly to the distal. The mesial side
of this root is decidedly flattened at its base, but as the center of the
surface is reached a shallow longitudinal groove is present, which is

Distolingual Groove

Fig. 97. — Upper First Molar, Distal
Surface.

156 ANATOMY.

continued to the region of the apex. The distal side is also possessed
of a similar groove, which extends throughout its entire length. Both
the buccal and lingual sides of the root are smoothly convex, the latter
being only about half the width of the former.

The distobuccal root (Fig. 97) is much the smallest of the three, and,
while inclined to flatness on its mesial and distal sides, it is much more
rounded than the mesial root. The mesial side is provided with a slight
longitudinal groove, and in rare instances a similar groove exists on the
distal side. The buccal and lingual sides are similar to those of the
mesial root. The root is generally straight, and tapers gradually from
base to apex, ending in a rounded point.

The lingual root (Fig. 97) is usually the largest and longest of the
three, and is more rounded in form than either of the buccal roots. The
lingual surface is inclined to flatness near its base, and is provided with
a well-defined longitudinal groove, which is sometimes independently
formed, while at others it is present as a continuation of the lingual
groove. This root being the only one given off from the lingual side of
the tooth, is constructed with a mesiodistal measurement about equal
to that of the base of the crown at this point. From its place of beginning
it passes first in a lingual and then in a buccal direction, forming a long
curve and ending in a sharp-pointed apex.

Bilious Type. — The upper first molar of this temperament is
manifest by a crown with angles well produced, the marginal ridges and
cutting-edges of the cusps bold and well marked. The cusps are of
medium length, with summits angular and pointed. The developmental
grooves are deep and often sulcate, and numerous supplemental grooves
are found upon the occlusal surface. The longitudinal and transverse
measurements of the crown are about equal, and when viewed upon the
occlusal surface, the angular nature of its anatomy is noted as a distinc-
tive feature. The neck is fairly well developed, giving a slight bell-
shape to the crown. The cervical line is made up of angles rather than
curves, and the roots are long and straight.

Nervous Type. — Like the teeth previously described under this
class, the crown of this tooth is of greater longitudinal than transverse
extent; the neck is especially well formed, producing a decided bell-
shape to the crown. The cusps are long and penetrating, the marginal
ridges sharply defined, as are also those ridges upon the central incline
of the cusps. The grooves of development are decided and frequently
sulcate. The buccal surface is rounded and smooth, with the buccal
groove extending well toward the cervical line. The lingual surface

UPPER FIRST MOLAR. 1 57

also presents a general convexity, and is usually divided by the lingual
groove. The mesial surface is convex over its cervical third, and the
occlusal margin is a decided convex ridge, serving as a point of contact
for the adjoining tooth, and thus forming the characteristic V-shape
common to this temperament. It is in this type that the fifth cusp is
most frequently present. The cervical line is much curved, and the
roots are slim and frail.

Sanguineous Type. — The crown of the upper first molar of this
type usually presents a slightly greater longitudinal than transverse
extent. The angles of the crown are poorly formed, being rounded and
smooth. The cusps are of moderate length, and are rounded in their
nature; the marginal ridges, as well as those ridges of the central incline
of the cusps, are less distinct than either of the forms previously described.
The buccal and lingual surfaces are convex and seldom broken by grooves;
the mesial and distal surfaces are convex in every direction, throwing
the point of contact with adjoining teeth near the center of the surface.
The roots are inclined to be large and oval in form, while the cervical
line is a series of long curves.

Lymphatic Type. — In this temperament the crown is much less
in its longitudinal than transverse measurement. The neck of the tooth
is poorly defined, the crown passing into the root-base without a marked
constriction. The mesial and distal surfaces are flattened and nearly
parallel with each other, providing a broad contact surface. The buccal
and lingual surfaces each present a marked general convexity, the latter
being frequently broken by the distolingual groove. The tooth is
provided with cusps which are short and heavy-set; the marginal ridges,
as well as all the ridges common to the occlusal surfaces, are poorly defined.
The developmental grooves are shallow and terminate abruptly. There
is but little curvature to the cervical line, and the roots are short, heavy-
set, and inclined to cluster together.

i5«

ANATOMY.

UPPER SECOND MOLAR.

&-& F f

7th yeat 8th year 9th year

10th year 11th year 12th year
Fl(i. 98.

1 6th year

Calcification Begins, from Four Centers, about the Fifth Year.
Calcification Completed, Sixteenth to Eighteenth Year
Erupted, Twelfth to Fourteenth Year.
Average Length of Crown, .28.

Average Length of Root, .51.

Average Length Over All, 79.

Calcification in this tooth takes place in precisely the same manner
as that of the first molar, but the formative process is much later in begin-
ning, the lime-salts commencing to accumulate in the four separate lobes
about the fifth year. At the beginning of the sixth year the formation
of the cusps is completed, soon after which they coalesce and the occlusal
surface of the crown is established. At the beginning of the eighth
year fully two-thirds of the crown is calcified, and the following year
the crown and neck are completed and the root-base outlined. By the
tenth year the beginning of separate root-development is observed;
at the twelfth year, or at the time of eruption, the roots are formed to
about one-half of their completed length, the process continuing until
the sixteenth or seventeenth year, when calcification is completed and
the root apices formed (Fig. 98). In many respects this tooth closely
resembles the first molar previously described, the crown presenting
the same number of surfaces similarly named, and also being provided
with the same number of roots. Notwithstanding this fact, there are
a number of ways in which they are at variance. The crown of the
second molar is smaller than that of the first, and the quadrilateral
outline common to the first molar is much compressed and broken in
the second. The distal cusps are much smaller proportionately than
the mesial cusps, this being particularly true of the distolingual cusp.

UPPER SECOND MOLAR.

J 59

This reduction in size of the distal cusps gives to that portion of the
occlusal surface a slight distal incline.

Occlusal Surface of the Crown (Fig. 99). — The general contour
of the' crown is best studied by viewing it directly upon the occlusal
surface; this aspect also shows to best advantage the difference in form
between this and the first molar tooth, as shown in figure 93. The
mesial and lingual outlines closely resemble the corresponding outlines
on the first molar, but the buccal and distal are much at variance, the
former passing into the latter without a distinct line of demarcation
existing between the two, this gradual blending of one into the other
being at the expense of the distobuccal angle of the crown, which is poorly

Buccal Groove

Distobuccal Cusp

Distal Fossa

Distolingual Groove

Mesiobuccal
Triangular Ridge

Mesial Groove

Mesiomarginal Ridge

Mesiolingual Cusp

Central Distal Incline of
Fossa Mesiolingual Cusp

Fig 90 — Upper Second Molar, Occlusal Surface.

developed. The crown is much compressed in a distobuccal-mesio-
lingual direction, making this measurement of the occlusal surface about
one-third less than the mesiobuccal-distolingual measurement. The
cusps are much inclined to cluster toward the center of the surface, this
being especially true of those on the lingual half.

Marginal Ridges of the Occlusal Surface (Fig. 99). — Like the occlusal
surface of the upper first molar, this surface of the second molar is
bounded by four marginal ridges — the mesial, distal, buccal, and lingual.
They are usually less marked than those found on the first molar, and
are much more variable in their individual anatomy. The mesiomarginal
ridge extends from the summit of the mesiobuccal cusp to the summit
of the mesiolingual cusp. It is concave in the direction of the body of
the crown, and is broken near its central portion by the mesial groove.
In some instances one or more small supplemental grooves are found

l6o ANATOMY.

to cross it. Compared with the mesiomarginal ridge of the first molar,
its length is much less and the convexity not so pronounced. The
distomarginal ridge, owing to the variation in form and size of the distal
cusps, is difficult to describe definitely; suffice it to say that it extends
from the summit of the distobuccal to the summit of the distolingual
cusp. The concavity is V-shaped, and is usually crossed near the
center by the distolingual groove. In some instances the distolingual
cusp is almost wanting, in others the distobuccal is but little developed;
when either of these conditions is present, the marginal ridge is extended
either to the buccal or to the lingual, in a measure taking the place of
the missing cusp. The mesial half of the buccomarginal ridge closely
resembles the corresponding margin of the first molar; beginning at
the mesiobuccal angle it ascends to the summit of the mesiobuccal
cusp, after which it descends by a longer incline to the buccal groove.
The distal half of the ridge, unlike that of the first molar, presents much
variety, its form being controlled by the character and position of the
distobuccal cusp, usually small. As most frequently observed, it ascends
to the summit of the cusp, and in so doing it presents a decided lingual
inclination. In passing down the distal incline the lingual inclination
is increased and gradually passes into the distomarginal ridge. Branch-
ing off from the mesiomarginal ridge, the linguo marginal ridge ascends
to the summit of the mesiolingual cusp and descends by a much shorter
incline to the distolingual groove. This portion of the margin is thrown
well toward the center of the surface, the location of the cusp carrying
it to that point. Like the distal half of the buccal margin, the outline
of the distal half of this margin is controlled by the position and form of
the distolingual cusp. In the majority of cases, when the cusp is moder-
ately strong, the ascent from the distobuccal groove to the summit of
the cusp is short and abrupt, the descent being somewhat more gradual,
and with a decided buccal inclination it passes into the distomarginal
ridge, or ends abruptly at the distal end of the distolingual groove.

The Cusps and Ridges (Fig. 99). — This tooth is provided with
four lobes or cusps, two of which are located on the buccal, and two on
the lingual side. They are usually smaller and less angular than the
cusps of the first molar. This is particularly true of both the distal
cusps, and especially of the distolingual cusp, which is often quite diminu-
tive and occasionally entirely wanting. When this latter condition
exists, the lingual half of the surface is for the most part occupied by
what would otherwise be the mesiolingual cusp, the absence of the
distal cusp permitting the distolingual groove to occupy a position near

UPPER SECOND MOLAR. l6l

the extreme distolingual angle, that portion of the surface which is
distal to the groove being a portion of the distomarginal ridge.

The Mesiobuccal Cusp (Fig. 99). — Like the corresponding cusp of
the first molar, this cusp is usually the longest of the four, and in many
instances covers a greater extent of surface than any of the others. Its
base is outlined by the buccal and mesial grooves, the two together form-
ing the mesiobuccal triangular groove. Descending from its summit
to the buccal surface is the mesiobuccal ridge; the marginal ridge makes
a double descent, one in a mesial and one in a distal direction, while
sloping toward the central fossa is the mesiobuccal triangular ridge.
The cusp is seldom traversed by supplemental grooves such as are found
on the corresponding cusp of the first molar.

The Distobuccal Cusp (Fig. 99). — As previously stated, this cusp is
not constant in its form; in some instances it is bold and well produced,
corresponding closely to the mesiobuccal cusp just described. When
thus pronounced it is possessed of ridges, and bounded by grooves which
are similar to those described in connection with the first molar. More
frequently the cusp is much rounded, its summit being carried well
toward the center of the surface. When this formation exists, the buccal
ridge is absent, the marginal ridges short and rounded; the distobuccal
triangular ridge which descends from it toward the center of the crown,
is short and heavy set.

The Mesiolingual Cusp (Fig. 99). — In the majority of instances this is
the largest cusp, particularly when there is a degenerate tendency in the
distolingual cusp. Descending from its summit are a number of ridges,
the marginal ridges being given off as already described, the mesiolingual
triangular ridge descending the central incline to the central fossa, and when
the cusp has an additional mesiodistal extent by the presence of a diminutive
distal cusp, other ridges descend in the same direction. The lingual
descent of the cusp is smooth and more rounded than the corresponding
surface of the first molar, and is seldom elevated in the form of a definite
ridge. The central outline of this cusp is marked by the mesial, the
distal, and the distolingual grooves.

The Distolingual Cusp (Fig. 99). — In no other cusp do we find such
a diversity of form as in the distolingual cusp of the upper second molar.
In some instances it is fully as prominent as its neighbor just described,
in others appearing as a mere fold of enamel, and it is not uncommon
to find it entirely wanting, the distomarginal ridge extending to occupy
a portion of the space which it should claim. Deductions might be
drawn from an average between these two extremes, wherein the existing

1 62 ANATOMY.

cusp would be much smaller than any of the others, the summit rounded
rather than sharp, but with a decided inclination to occupy the extreme
distolingual angle of the surface, in this latter respect differing from the
distobuccal cusp. The mesial and buccal outlines of the cusp are formed
by the distolingual groove, and its mesiolingual incline contributes to
the formation of the distal fossa.

The Fossae and Grooves of the Occlusal Surface (Fig. 99). —
These in name, number, and general form are similar to those of the
first molar. The central fossa is never, strictly speaking, in the center
of the surface, and is formed by the central incline of the mesiobuccal,
distobuccal, and mesiolingual cusps. It is seldom so deep as the central
fossa of the first molar. The distal fossa is more or less pronounced,
its size and position being controlled by the extent of development in
the distolingual cusp. The distolingual groove, which usually crosses
the lingual surface of the first molar near its center, is not constant in
its location on this tooth, in some cases being near the center, in others

near the distolingual angle of the crown. The
buccal groove is never constant in its location,
usually crossing the buccomarginal ridge and pass-
ing over the buccal surface near its mesiodistal
center, but it is not uncommon to find it forced to
the distal by a diminution in the size of the distal
cusp.

Buccal Surface of the Crown (Fig. 100). —
The most constant difference between this and the
corresponding surface of the upper first molar is
the wandering location of the buccal groove.
While in the majority of instances it may be found
,, ,- near the mesiodistal center of the surface, it is not

Fig 100. — L p p e r

Second Molar, But- uncommon to find it passing over the distal third,
or even as far posterior as the distobuccal angle.
In general, the surface is somewhat more convex and necessarily less
extensive than the buccal surface of the first molar. The buccal ridges
which descend from the summit of the two buccal cusps are seldom so
marked as those on the first molar, and in many instances the distal
ridge is wanting. The distal half of the surface frequently passes
into the distal surface, by a long gradual sweep, there being no line of
demarcation between the two.

The Lingual Surface of the Crown (Fig. 101). — In keeping with
the other surfaces just described, the lingual surface differs from the

UPPER SECOND MOLAR.

163

Lingual
Root

Mesial In-
cline of

Mesiolin-
gual Cusp

Cervica
Line

Disiomar-
ginal Ridge

Fig. ioi.

Lingual Groove, ending
in Lingual Pit

-Upper Second Molar, Lingual
Surface.

corresponding surface of the first molar in that it presents a greater
general convexity. This is particularly true in passing from the cervical
line to the occlusal surface. The lingual groove is also less constant
in its location. In most in-
stances it is to be found a
little to the distal of the cen-
ter, in others being as far
posterior as the extreme distal
third of the surface, and in
rare instances it is entirely
wanting. The general char-
acter of this surface, which is
smooth and convex, is seldom
broken by the presence of
well-defined ridges, such as
are usually found descending
from the lingual cusps of the
first molar. As previously
referred to, the mesial, distal, and buccal surfaces, as well as the surfaces
under consideration, are proportionately smaller than those of the first
molar, and, while this refers to both the transverse and longitudinal
measurements, it is particularly applicable to the latter.

The Mesial Surface of the
Crown (Fig. 102). — Aside from
this surface being of less extent
than the corresponding surface of
the first molar, there are no other
differences of importance. In
many instances, however, there
is a decided tendency for the
surface to be concave from
buccal to lingual, the convex
distal surface of the first molar
closely fitting into this concavity.
Another variation which is fre-
quently observed is that of the
longer and more gradual sweep which it takes in passing into the
lingual surface.

The Distal Surface of the Crown (Fig. 103).— This differs from
the distal surface of the first molar principally in its more pronounced

Lingual
Root

Cervical
Line

Mesiolin-
gual Angle

Mesiobuc-
cal Root

Fig. 102.

Mesiobuc-
cal Angle

-Upper Second Molar, Mesial
Surface.

164

ANATOMY.

Distobuc-
cal Root

Mesiobuc-
cal Root

Lingual
Root

Distoling-
ual Groove

convexity. Its general form is also much influenced by the nature of
the two distal cusps. If one or the other of these is sparingly developed,
either the buccal or lingual half of the surface, as the case may be, is
quickly rounded off to pass into the deficient lobe.

The Angles of the Crown. — The increased inclination for the
crown of this tooth to general convexity dispels, in a measure, the presence
of angles, as such, in correspondence with the four corners of the first
molar. In some instances the crown is represented as a fairly well-
formed quadrilateral, in which case the angles are well defined, but

usually this outline is so much
broken by a mesiodistal com-
pression that the angular form
of the crown is entirely abolished.
But, whatever the form of the
crown may be, it is well to ad-
here to the commonly accepted
term, and speak of that point at
which the sides of the crown
unite as the angles, each being
named in accordance with its
location.

The Neck of the Tooth.—
The principal variation between
the neck of this tooth and that
of the first molar, is that produced by the greater general convexity of
the crown, which contributes to ; the production of a neck much more
constricted. There is also a greater variety in the contour of the neck,
incident to the variation in the general outline of the crown.

The Roots of the Upper Second Molar. — These are the same
in name and number as those of the first molar — two buccal and one
lingual. In many respects they differ from the roots of the first molar.
They are much smaller, frequently inclined to cluster together, and are
often fused, in some instances, all three being united, in others the union
existing between but two. When isolated, each root usually presents a
decided distal curve near its apical third. When the crown is flattened
from mesial to distal, as before described, the distobuccal root is forced
to occupy a position much more to the lingual than that assumed by
the mesiobuccal root. The lingual groove seldom passes over the lingual
root, as observed on the first molar.

Fig. 103.

-Upper Second Molar, Distal
Surface.

UPPER THIRD MOLAR. 165

UPPER THIRD MOLAR.

10th year nth year 12th year 14th year 1 8th year

Fig. 104.

Calcification Begins, Ninth Year.

Calcification Completed, Eighteenth to Twentieth Year.
Erupted, Seventeenth to Twentieth Year.
Average Length of Crown, .24.

Average Length of Root, .44.

Average Length over All, .68.

Calcification of this tooth takes place in precisely the same manner
as that in the first and second molar, with the exception of the number
of lobes, which are sometimes three and sometimes four. The lime-
salts begin to accumulate between the eighth and ninth year, and con-
tinue with somewhat more activity than that of the first and second molar.
Between the ninth and tenth year the three or four cusps, of which the
future tooth is to be composed, have coalesced, and by the eleventh year
calcification in the crown of the tooth is completed; at the end of the
following year the roots, which are variable in number, have made
considerable progress; at the fourteenth year they are calcified to about
half their length, while at a period between the eighteenth and nineteenth
year the formative process is completed (Fig. 104). This tooth, like
the cuspid, is usually fully formed before eruption takes place.

This tooth is subject to a greater variety of form than any other;
in rare instances it is similar in general outline and cusp formation to
the first molar, but in a vast majority of cases it is dissimilar, the most
constant deviation being its size, which on the average is about one-
third less. In the accompanying illustration (Fig. 105) the forms most
frequently met with are shown. It will be observed that the contour
of the tooth in general is much more rounded than either the first or second
molar. The buccal angles of the crown are alone well marked, the

1 66

ANATOMY.

mesial and distal surfaces passing into the lingual surface by a long,
gradual sweep, and thus obliterating the lingual angles. In many
instances the tooth is tritubercular, and is usually made so by the absence
or diminutive size of the distolingual cusp. Just as this cusp was inclined
to degenerate in the second molar, so we find this retrograde develop-
mental tendency increased in the third molar. With this change in the
construction of the occlusal surface, there is a corresponding variation
in the grooves, ridges, and fossae.

Fig. 105. — Various Types of Upper Third Molar.

Mesial Surface of the Crown (Fig. 106). — In many particulars
this surface corresponds in form and outline to the mesial surface of the
first molar; it is, however, usually much more convex, seldom presenting
a concavity or even a positive flatness. The surface is not only rounded
from buccal to lingual, but also from the cervical line to its occlusal
margin. Thus formed, a point of contact is provided near the center
of the surface. The occlusal margin, the buccal margin, and the cervical
margin are almost identical to those of the first molar, but in most in-
stances the lingual margin is wanting, the surface gradually passing
into the lingual without a decided line of demarcation.

Distal Surface of the Crown (Fig. 107). — This surface is much
less extensive in comparison to the size of the crown than the correspond-
ing surface of either the first or second molars. It is decidedly rounded

UPPER THIRD MOLAR.

167

Buccal
Boot

Mesiobuc-
cal Cusp

Lingual
Root

Mesial Groove
Fig. 106. — Upper Third Molar, Mesial
Surface.

in every direction and is frequently crossed by the distal developmental
groove, and sometimes by one or more supplemental grooves. The
general form of the surface is much influenced by the presence or absence
of the distolingual cusp; with the
former, the surface is more exten-
sive, presenting less convexity and
resembling more closely the distal
surface of the first and second
molars; with the latter, the extent Mesiobuc

' cal Root

of the surface is decreased and
the convexity increased.

Buccal Surface of the
Crown (Fig. 108). — The mesial
portion of this surface is in no
way at variance with the mesial
portion of the buccal surface of
the first or second molar, but
much variety of form exists in

the distal portion. The buccal groove which serves to separate these
two portions is located well toward the distal third of the surface, thus
reducing the size of the distal portion to about one-third that of the
mesial portion. In general, the surface is but little more convex than

the corresponding surface of the
first and second molar. Its mesial
border is definitely outlined, as
are also the cervical and occlusal
margins, but the distal margin
cannot be definitely located, the
surface tending to pass gradually
into the distal surface. Like the
distal, the extent of this surface
is much regulated by the size and
shape of the distobuccal cusp,
which, like the distolingual cusp,
is inclined to degenerate.
Lingual Surface of the Crown (Fig. 109). — Like the distal surface
previously described, the form of this surface is much influenced by the
presence or absence of the distolingual cusp. When this cusp is wanting
or but little developed, the surface presented is decidedly convex and
smooth; in many instances the mesiodistal curvature described is almost

Distobuccal
Root

Buccal
Groove

Lingual
Root

Distomar-
ginal Ridge

Mesiolin-
gual Cusp

Fig. 107. — Upper Third Molar, Distal
Surface.

1 68

ANATOMY.

Mesiobuc-
cal Root

Mesiobuc-
cal Cusp

Distobuccal
Root

Distal
Groove

Distobuccal
Cusp

Fig 108

-Upper Third Molar, Buccal

Surface.

a perfect semicircle, and in passing from the cervical line to the occlusal
margin the surface is carried well toward the center of the crown by
a long gradual sweep toward the lingual. The lingual groove is usually
absent. The change in form produced ,by the presence of the disto-

lingual cusp is principally
noticeable in a less pro-
LinguaiRoot nounced convexity and the
presence of the lingual
groove, which may be
noticed as a slight depres-
sion or as a well-defined
groove. This groove, when
present, is always located
near what would represent
the distolingual angle of the
crown; the distolingual cusp
seldom if ever being of suf-
ficient size to force its location near the center of the surface as in the
first molar. In some instances this groove is shown upon the lingual
surface when the cusp is not present; in this case the distomarginal
ridge represents in a manner the cusp by its bold, heavy development.

Occlusal Surface of the Crown (Fig. no).— When looking directly
upon this surface, an opportunity is presented
to study the general contour of the crown; the
most noticeable difference in this respect be-
tween this tooth and the first molar being ob-
served in its smaller size, and the absence of
well-marked angles. It will be noted that the
mesial and buccal outlines in a measure resem-
ble the corresponding outlines of the first and
second molars, but there is scarcely any similarity
existing when comparing the distal and lingual
outlines. In some instances the crown is tri
angular (Fig. 105) ; in others the mesiobuccal and
bucco-marginal outlines form an obtuse angle,
the free ends of which are joined together by a
long semicircle, the latter constituting the distal and lingual outlines (Fig.
105). Again, almost the reverse of this last-mentioned form is seen, the
buccal and distal outlines constructing the angle, while the semicircular
connection between the two is made up of the mesial and lingual outlines.

Fig. 109.—
Third Molar,
Surfac e.

U p p e r
Lingual

UPPER THIRD MOLAR.

169

Buccal Buccomar-
firnnve ginal Ridee

Distomar-
ginal Ridge
Distal
Fossa
i)istolin-
gual Cusp

The Marginal Ridges. — The me siomar ginal ridge is usually well
defined, and in most instances is crossed near its center by the mesial
groove, and frequently by two or more supplemental grooves. This
marginal ridge is probably the most constant in form, the numerous
variations to which the surface is liable seldom making any material
alteration in it. Unlike the ridge above described, the distomar ginal
ridge is most variable in its construction, nearly all of the forms charac-
teristic of the occlusal surface exerting a controlling influence over it.
In rare instances the ridge resembles that of the first and second molars,
but this form is most frequently interfered with by the absence or diminu-
tive size of the distolingual cusp, the ridge itself frequently supplying the
place of the cusp. In many
cases the ridge is elevated near
its central part by being rein-
forced by a portion of the
oblique ridge. When the dis-
tolingual cusp is wanting, this
ridge not infrequently de-
scends from the summit of
the distobuccal cusp to the
distal groove and from this
point ascends obliquely to the
summit of the lingual cusp.
The bucco-mar ginal ridge may be described as similar in most respects to
the corresponding margin on the first and second molars, the principal vari-
ation being in the distal half, which is much shorter and less pronounced.
In the linguomar ginal ridge, again, much variety in outline is noticeable.
In nearly all instances the ridge is thrown much nearer the center of the
body of the crown, and, when the tooth is bicuspid in form, it simply
makes a mesial ascent of the lingual cusp, followed by a gradual incline,
and passes into the distal ridge, as .above noted. When the distolingual
cusp is present, the ridge is similar to that upon the first and second
molars, with the exception of the distal portion, which is less clearly
marked.

The Cusps (Fig. no). — As previously stated, the form most frequently
met with is tritubercular, two of the cusps being upon the buccal and
one upon the lingual half of the surface.

The Mesiobuccal Cusp (Fig. no). — This cusp corresponds in nearly
every particular to the mesiobuccal cusp of the first and second molars;
it is the most constant in size and form of the three. Its summit is usually

Mesiobuc-
cal Cusp
Central
Fossa

Mesiomar-
ginal Ridge

Fig.

-Upper Third Molar, Occlusal
Surface.

1 70 ANATOMY.

angular, and the numerous ridges which descend from it are well defined
and similar in name and number to those of the first molar.

DistobuccalCusp (Fig. no). — The constant inclination to degeneracy
in the distal portion of the crown of the tooth is noticeable in this cusp,
which is much smaller than the mesiobuccal and scarcely half as large
as the corresponding cusp of the first and second molars. In some
instances, however, it is inferior only in size, retaining its angularity,
being possessed of small but well-defined ridges.

The Lingual Cusp (Fig. no). — When the three cusps alone are
present, this one is much the largest, the extent of the surface covered

being all of the lingual half of the crown. The
summit of the cusp, which is thrown well toward
the center of the body of the crown, is prominent,
but seldom angular. Only in rare instances will
there be found a lingual ridge descending there-
from, but the central incline is usually marked
by a number of wrinkles or folds of enamel re-
sembling minute ridges. The central boundary
of this cusp is marked by the mesial and distal
Fig. in.— Upper Third developmental grooves.

Molar, Occlusal Surface, _, . .. . _, ._. . T . it

with Distoiingual Cusp The Distolingual Cusp (Fig. no).— It is the

and Distal Fossa Poorly p resence or absence of this cusp that contributes

most to the variations present in the crown.
When present, it is usually diminutive in size, and is without definite
form. In many instances nature is apparently attempting to cast it off
in precisely the same manner in which she is attempting to add to the
first molar by a development of the "fifth cusp," the distobuccal cusp
appearing to hang to the distolingual angle of the crown in a manner
very similar to the "fifth cusp." W r hen thus situated, it is separated
from the body of the crown by a groove, which cannot be considered as
being upon the occlusal surface. When located in its normal position,
it has for its inner boundary the distolingual groove.

The Fossae and Grooves of the Occlusal Surface. — The great
variety and form common to this surface exerts a controlling influence
over the size, number, and position of the grooves and fossae. In the
tritubercular class the central fossa alone is present. The developmental
grooves, with the exception of the buccal, are not definitely outlined,
but, descending toward the fossae from the central incline, are numerous
small ridges divided from each other by a like number of diminutive
supplemental grooves. The distal groove is sometimes well defined,

UPPER THIRD MOLAR. 171

and crosses over the oblique ridge, which in this type becomes the disto-
marginal ridge. When the distolingual cusp is present, all of the ridges
and grooves are more pronounced. In this case the central fossa cor-
responds more closely to the central fossa of the other molar teeth, this
resemblance increasing just in proportion as the size of the distolingual
cusp increases. The distal fossa, in a vast majority of instances, is
present as a mere pit; the size of this fossa is likewise much controlled
by the extent of development in the distolingual cusp. Where the disto-
marginal ridge is supplementary to the distolingual cusp, the distolingual
groove lies between the former and the oblique ridge. Another peculi-
arity found only upon the occlusal surface of this tooth is, what appears
to be an effort upon the part of the cusps to cluster toward the center.
This is common only to those teeth possessing three cusps, and accompany-
ing this form the central fossa shows a number of fantastically arranged
grooves and ridges which ascend the cusps, passing over the marginal
ridges and breaking them into a number of small tubercles.

Temperamental Types. — The third molar tooth is probably less
influenced by the character and habits of the individual than any other
tooth in the mouth. The inclination to a general degeneracy is no doubt
favored by civilization. With a constant decline in the functional
activity, brought about by the present culinary methods common to
civilization, this tooth in a measure becomes useless, and nature is
gradually making an effort to cast it off. While there are undoubtedly
many individuals possessed of the highest mental attainments with the
third molar as fully developed as either the first or the second, this con-
dition is usually confined to those possessed of little intellectuality. If,
in general, the temperament of the subject be taken into consideration,
the cusp-formation on this tooth will correspond in a relative degree
to that on the bicuspids and molars.

CHAPTER IX.

A Description of the Lower Teeth in Detail. — Calcification, Erup-
tion and Average Measurements. — Their Surfaces, Ridges,
Fossae, Grooves, Sulci, etc.

THE LOWER TEETH.

In most respects the anatomy of the lower teeth is similar to that
of the upper, but in each class we find a slight variation existing between
the two sets. As compared to the upper incisors, the crowns of the
lower incisors are more slender and somewhat more angular in outline.
The roots are more slender, proportionately longer, more flattened
laterally, and seldom crooked. The crowns of the lower incisors
are probably more constant in form than those of any other teeth,
seldom varying except in size. The mesiodistal measurement of the
crown of the lateral incisor is a trifle greater than that of the central, a
condition exactly the reverse to that of the upper incisors. The labial
and the lingual surfaces of these teeth are smooth, and, with the exception
of young teeth, show but little trace of the developmental process by
the presence of grooves, fissures, etc. The outline of the lower cuspids
is almost identical to that of the corresponding teeth in the superior arch,
excepting that they are in every way more slender. The bicuspids are
proportionately smaller in every direction than those of the upper jaw,
their cusps are smaller, and they are seldom found with more than one
root. The crowns of the lower molars are somewhat larger than those
of the upper, and are provided with five cusps instead of four,* and they
are attached to the alveolus with two, instead of three, roots. In the
incisors, cuspids, and bicuspids the process of development is the same
as in the corresponding upper teeth, calcification taking place from the
same number of centers along the coronal extremities. In the molars,
however, development may proceed from five centers instead of four,
as in the upper molars. The manner of development, and the period
at which this action takes place, so nearly corresponds with that of the
upper teeth that the process will not be repeated.

*The lower first molar has five cusps in ninety per cent, of cases, while in the second
five cusps are present in about fifty per cent.

I 7 2

LOWER CENTRAL INCISOR.

*73

LOWER CENTRAL INCISOR.

Labial
Grooves

Distal
Angle

Calcification Begins, First Year after Birth.

Calcification Completed, about the Tenth Year.
Erupted, Seventh to Eighth Year.
Average Length of Crown, .34.

Average Length of Root, .47.

Average Length over All, .81.

Like the upper central incisor, this tooth presents for examination
four surfaces, a cutting-edge, and various angles, margins, etc. By the
union of the labial and lingual surfaces at the cutting-edge the incisive
feature is established and the double incline plane common to incisors
produced.

The Labial Surface of the Crown (Fig. 112). — This surface is
smooth and convex, its general outline resembling an inverted cone,
the base of which is formed by the cutting-edge and the apex by the
cervical line. The margins of the surface
are, with the exception of the cutting-edge,
not so well defined as those of the upper
central. Near the cutting-edge the mesial
and distal margins pass somewhat abruptly
into the respective lateral surfaces, but as
the neck of the tooth is approached they are
much rounded. The incisive marg"
squarely cut, and is nearly at right angles
with the long axis of the tooth. The cervical
margin is fairly well defined, and is deeply
concave in the direction of the root. Except
in very young teeth, this surface is seldom
much broken by the labial grooves; but in
certain types one or more transverse ridges
may be found occupying the cervical third.
The mesiodistal diameter at the cutting-
edge is about one-third greater than at the cervical line, and, while
these measurements are likely to vary in accordance with the tempera-
ment of the subject, this variation is not so pronounced as in the upper
incisor. The surface is frequently inclined to flatness near the incisive
margin, the general convexity becoming more marked as the cervical
line is approached.

The Lingual Surface of the Crown (Fig. 113). — In general out-
line this surface resembles the labial, with the exception of the cervical

IS Cervical
Line

Mesial
Angle

Cervica
Ridge

Fig. 112. — Lower Incisor, Right
Side, Labial Surface.

174

ANATOMY.

Lingual Grooves

Distomar-
ginal Ridge

Cervical
Ridge

Mesiomar-
ginal Ridge
Lingual
Fossa

Fig. 113. — Lower Incisor, Right Side,
Lingual Surface, Strongly Developed.

margin, the lines of which are somewhat more acute. The surface
presents a marked concavity from the cutting-edge to the cervical ridge,
and also a slight transverse concavity near the incisive margin. All of

the margins are more definite than
those of the labial surface. The
mesial and distal margins are formed
by the marginal ridges common to
these borders, but these ridges are not
so well defined as those of the upper
incisors. The cervicomarginal ridge
is present as a well-rounded band of
enamel, but is never a well-defined
cingulum, or cuspule. The depres-
sion between these marginal ridges is
so slight that it can scarcely be re-
ferred to as a fossa, although usually
characterized as the lingual fossa.
The lingual grooves are generally more pronounced than the corres-
ponding developmental grooves of the labial surface, but end more
or less abruptly before reaching the cervical ridge. The mesiodistal
measurements of the surface are a trifle less than the corresponding
measurements of the labial surface. cutting-edge

The Mesial Surface of the Crown (Fig.
114). — The outline of this surface is exactly
the reverse of the labial and lingual just de-
scribed, being a cone, with its base directed
downward or in the direction of the root,
while its apex is formed by the mesial ex-
tremity of the cutting-edge. The cervical
margin of the surface, or that represented by
the base of the cone, is concave; the labial
and lingual margins are rounded over the
cervical third and inclined to angularity near
the cutting-edge. There is a slight convexity
over the entire surface, which is most marked
near the center. The lingual half of the
cervical portion slopes away to the distal,
passing gradually into the lingual surface. By the union of this surface
with the cutting-edge and the labial and lingual surfaces, the mesial

Cervical

Line
Cervical

Ridge

Fig. 114. — Lower
Incisor, Mesial Sur-
face.

angle of the crown is formed.

This angle is well outlined and reaches

LOWER CENTRAL INCISOR.

175

Lingual
Ridge
Cervico-
lingual
Ridge

Labial
Ridge
Cervical
Line

Fig. 115. — Lower Incisor, Distal
Surface.

out toward the median line, giving to this portion of the crown a promi-
nent appearance. Near the cutting-edge the surface presents a slightly
rounded prominence, which provides a
point of contact with the corresponding
tooth of the opposite side.

The Distal Surface of the Crown
(Fig. 115). — In a general way this sur-
face closely resembles that of the opposite
or mesial side of the crown. Near the
cutting-edge the surface is usually more
prominent and presents a more marked
convexity, and near the cervical margin
it is flattened and sometimes slightly con-
cave. The union of this surface with the
cutting-edge and the labial and lingual
surfaces forms the distal angle of the
crown, which, like the mesial angle, is

square and well defined. The margins of the surface are in no
different from those of the mesial surface.

The Cutting-edge. — In the young tooth this incisive

is thin and generally divided into three distinct parts
(Fig. 116) by the developmental grooves, but these
disappear so early that they can scarcely be considered
in connection with a description of the fully developed
tooth; in fact, the cutting-edge of this, as well as that
of all the lower incisors, is so susceptible to change by
mechanical abrasion that a normal condition is of but
short duration. After the disappearance of the
primitive cusps, and before further abrasion has taken
place, the edge is fairly sharp and placed nearly at
right angles with the crown. As in the upper incisors,
the cutting-edge is in a line with the long axis of the
tooth. The labial margin of the edge is slightly con
vex, while the lingual is irregularly concave to the
same extent.

The Cervical Margin. — This marginal line,
which is marked by the extent of the enamel cap,
corresponds closely to that of the upper incisors,
dipping down with a graceful concavity on the labial and lingual surfaces
with a corresponding convexity on the mesial and distal surfaces.

way

margin

Developmental Grooves

Fig. 116. — Young
Lower Incisor, with
Cutting-edge, show-
ing the Lines of
Development.

176 ANATOMY.

The prominence of this enamel margin, together with the nature of
the curvature, is much influenced by the tooth type.

The Neck of the Tooth. — A distinctive feature of this tooth is
found in the convergence of its mesial and distal surfaces in passing
from the cutting-edge rootward, thus producing a neck much constricted
from mesial to distal. When examined from either the mesial or distal
surface, this feature is scarcely noted, the crown passing into the root
with little more than the cervical line as a mark of separation. The
labial and lingual portions of the neck are rounded and narrow, while
the two lateral sides are flat and broad.

The Root. — The root of this tooth is usually smaller than that of
any other tooth in the mouth. It is much flattened from mesial to distal,
while the labial and lingual aspects are rounded and narrow. Besides
being flattened and broad, the mesial and distal sides are usually found
with a longitudinal depression extending from a point near the base of
the root almost to its apex. These surfaces gradually taper from the
base to the apex, while the labial and lingual first widen from the base
and then gradually taper to the apex. The contour of the root-base
is generally reduced at the apical extremity, although in some instances
the latter is a rounded point. While the root of the tooth is usually
straight, there is sometimes a tendency for the apical third to have a
slight distal inclination.

LOWER LATERAL INCISOR.

Calcification- Begixs, First Year after Birth.

Calcification Completed, Tenth to Eleventh Year.
Erupts, Eighth to Ninth Year.
Average Length of Crown, .35.

Average Length of Root, .50.

Average Length over All. .85.

The crown of this tooth differs from the central incisor in being
broader from mesial to distal at the cutting-edge, resulting in a crown
more strongly bell-shaped. The cutting-edge, instead of being at right
angles to the long axis of the tooth, slopes to the distal at the expense
of the distal angle, which is much rounded, while the mesial angle closely
resembles the corresponding angle of the central incisors. The labial
and mesial surfaces do not differ materially from the corresponding
surfaces of the central incisor, excepting that the lingual more frequently
shows the lines of development, and the distal is at variance in having
that portion which contributes to the formation of the distal angle ex-
tended and prominent. The marginal ridges of the lingual surface are

LOWER CUSPID.

I 7 7

probably more definitely outlined than those of the central incisor, and
the crown in general presents a stronger appearance. The neck of the
tooth is similar to the neck of the central incisor, as is also the root, with
the exception of a slight addition to its length.

Labial Grooves

LOWER CUSPID.

Calcification Begins, Third Year after Birth.

Calcification Completed, Twelfth to Thirteenth Year.
Erupts, Twelfth to Thirteenth Year.
Average Length of Crown, .40.

Average Length of Root, .60.

Average Length over All, i.oo.

There is probably a greater similarity existing between the upper
and lower cuspid teeth than in any other class of teeth in the mouth.
Occupying as they do a prominent position in the dental arch, and being
called upon to perform the double function of incising and tearing the
food, their crowns are strong and heavy-set, and their roots long and
firmly anchored in the alveoli. Like the upper cuspid, the crown of the
lower is surmounted by a single
cusp, from the summit of which
descend a mesial and a distal
cutting-edge. There is also a
labial, lingual, mesial, and distal
surface presented for examination.

The Labial Surface of the
Crown (Fig. 117). — The crown of
the tooth, being a little longer than
that of the upper cuspid, gives to
this surface the appearance of be-
ing more slender, when in reality
there is but little difference in the
width of the two teeth. This sur-
face is smooth and convex, and,
while the labial grooves are usually

Mesial Cut-
ting-edge
Mesia
Argle
Labia
Ridge
Cervica-
Ridge

Distal Cut-
ting-edge

Distal Mar-
gin

Fig. 117. — Lower Cuspid, Labial Surface.

present, they are not so marked as those found upon the corresponding
upper tooth. A pronounced feature of the surface is the labial ridge,
which extends from the summit of the cusp to the cervical line, provid-
ing additional strength to the crown. Aside from this ridge and the labial
grooves, the surface is occasionally broken by one or more transverse
ridges over the cervical portion. The margins of the surface closely
resemble those of the upper cuspid, the incisive and mesial being definite

i 7 8

ANATOMY.

Lingual Grooves

Distal Angle

Distomar-
ginal Ridge

Fig. 118. — Lower Cuspid, Lingual
Surface

in character, while the distal is made equally indefinite by the passing
of the labial into the distal surface by a gentle curve.

The Lingual Surface of the

Crown (Fig. 118). — The ridges and

grooves of this surface are far less

bold in their character than those

Ridge a of the upper cuspid. The lingual

Mesiomar- . , , . , 1 • • j i r

ginai Ridge ridge, which divides the surface

Cervical

Ridge into two equal parts, extends from
the summit of the cusp to the base
of the cervical ridge, while the
marginal ridges pass rootward from
the angles of the crown, and, unit-
ing, form the cervical ridge. The
slight depressions between the
lingual ridge and the marginal
ridges correspond to the palatal
grooves of the upper cuspid, but
in this tooth partake more of the

nature of fossae.

The Mesial Surface of the Crown (Fig. 119). — A peculiarity

found in connection with this surface is the general plane existing between

the crown and root-surface. In all

other teeth the mesial and distal

surfaces are found to bulge some-
what beyond the corresponding

surface of the root, but this surface

of the lower cuspid is not only

usually in a direct line with the

mesial surface of the root, but is

occasionally inclined to the distal,

resulting in a crooked or bent ap-
pearance to the tooth. In addition

to this individual peculiarity, the

surface is flat and passes by a long

curve to meet the lingual surface.
The Distal Surface of the

Crown (Fig. 120). — This surface is

somewhat less in extent than the mesial surface, and, in place of being

flat and in line with the root-surface, that portion near the angle of

Labial
Ridge

Cervical
Ridge.

Lingual
Ridge

Mesiomar-
ginal Ridge

Cervical Line

Fig. 119.

-Lower Cuspid, Mesial
Surface.

LOWER CUSPID.

I 79

Lingual
Ridge

Cervicolin-
gual Ridge

Mesial
Groove

the crown presents a marked convexity, while that near the cervical line
is frequently slightly concave. This general form of the surface further
assists in producing the distal crook previously referred to. The lingual
margin is well defined and somewhat angular, while the surface passes
so gradually into the labial that a positive line of demarcation can scarcely
be said to exist.

The Cusp and Cutting-edges. — In most respects these are similar
to the corresponding parts of the upper cuspid. The length of the
mesial cutting-edge is usually somewhat less than that of the distal, but
this difference is seldom so marked
as that found in the upper cuspid.
The mesial and distal angles of the
crown are equally as pronounced
as those of the corresponding upper
tooth.

The Neck of the Tooth.—
This is shown by a fairly well-
marked constriction, but the pass-
ing of the mesial surface of the
crown into the mesial surface of
the root is not broken by this cir-
cular depression. On account of
this latter feature the neck of this
tooth is somewhat less pronounced
than that of the upper.

The Root of the Lower Cuspid. — The root of this tooth is some-
what shorter and more flattened on its mesial and distal sides than that
of the upper cuspid, this lateral flatness frequently amounting to a
decided longitudinal depression or groove. As referred to in the descrip-
tion of the crown, the mesial side of the root is continuous in a direct
line with this surface of the crown, but as the apical third of the root is
approached, there is frequently found a slight distal inclination which
affects alike both the mesial and distal sides. The labial and lingual
surfaces of the root are abruptly convex and taper very gradually from
the cervical line to the apex, while the mesial and distal surfaces taper
much more rapidly, the four ending in a slender apex usually flattened
from mesial to distal.

Fig. 120.

-Lower Cuspid, Distal
Surface.

i8o

ANATOMY.

THE LOWER BICUSPIDS.

In many respects these teeth are similar to the bicuspids of the
upper jaw, the chief differences being that they are somewhat shorter
and smaller in every respect. Their crowns are much more rounded and
the cusps are never so strongly developed. Unlike the upper bicuspids
the buccal and lingual cusps are connected by a transverse ridge. The
roots are much less flattened from mesial to distal, and are seldom
bifurcated.

LOWER FIRST BICUSPID.

Calcification Begins, about the Fourth Year.

Calcification Completed, Eleventh to Twelfth Year.
Erupted, Tenth to Eleventh Year.
Average Length of Root, .30.

Average Length of Crown, .54.

Average Length over All, .84.

In general, the crown of this tooth is much more rounded and
smaller in all its measurements than that of the upper bicuspid. The
buccal surface presents a much greater convexity, which results in
forcing the summit of the buccal cusp well toward the center of the long
axis of the tooth. The mesiodistal and buccolingual measurements of
summit of Buccal Cu the crown are nearly equal, and

about correspond to the max-
imum length of the crown. As
in the upper bicuspids, the de-
velopment of this tooth is similar
to that of the incisors and cus-
pids, the buccal cusp being de-
rived from three lobes, while
the lingual results from a single
center.

The Occlusal Surface of the Crown (Fig. 121). — This surface
is so unlike that of the corresponding surface of the upper first bicuspid
that a separate description without further comparative reference is
required. In general outline the form of a rounded triangle is approached,
the buccal margin serving as one side of the triangle, while, by the union
of the mesial and distal margins to form the lingual, the remaining sides
are established.

The Buccal Cusp. — As previously stated, the summit of this cusp
is thrown well toward the center of the surface. Descending from it

Buccal Ridge

Distal Pit

Buccal Groove
Mesial Pit

Triangular
Ridge

Lingual Ridge

FlG. 121. — Lower First Bicuspid,

Occlusal Surface.

THE LOWER BICUSPIDS.

181

Buccal Ridge

Cervical
Ridge

Bucca
Groove

Cervica
Line

Fig. 122. — Right Lower First Bi-
cuspid, Buccal Surface.

are four well-defined ridges — the buccal ridge to the buccal surface,

the mesial cutting-edge, the distal cutting-edge, and the triangular

ridge, the latter descending in a lingual

direction to meet the lingual ridge or

cusp. This ridge divides the surface

into two parts, the center of each being Di ^ b £, ccal

marked by a well-defined pit — the mesial

and distal pits. The mesial and distal

cutting-edges are frequently crossed by

the buccal grooves, and mark the line

of union between the central and two

lateral lobes of the buccal cusp. The

marginal ridges, one of which begins at

the mesial angle and the other at the

distal angle, pass to the lingual, where

they unite to form the Ungual ridge or

cusp.

The Lingual Cusp. — This cusp is

seldom well developed, and corresponds to the cervical ridge of the

incisors and cuspids. The extent of development in the lobe is ex-
tremely variable, in some instances amounting to little more than a

continuation of the mesio- and disto-
marginal ridges, while in others there is
a building-up of the enamel in the form
of a small tubercle. When this latter
condition is present, the triangular ridge
of the buccal cusp contributes to its
formation. The triangular ridge fre-
quently divides into two or more
smaller ridges, which usually end in the
mesial pit, but in some instances they
continue to the lingual, and divide the
lingual ridge into two or more smaller
tubercles.

The Buccal Surface of the Crown
(Fig. 122). — This surface is smooth and
convex in all directions, and in general
outline there is but little variation be-
tween it and the corresponding surface of the upper first bicuspid.

It is traversed from the point of the cusp to the cervical line with a

Buccal Cusp Triangular
Ridge

Distal Pit

Mesial Pit
Lingual
Ridge

Fig. 123. — Left Lower First Pi-
cuspid, Lingual Surface.

l82

ANATOMY.

Buccal Cusp

Lingual Cusp
or Ridge

Fig. 124 — Left Lower First Bicus-
pid, Mesial Surface.

rounded ridge, the buccal ridge, upon either side of which are the
buccal grooves.

Lingual Surface of the Crown (Fig. 123). — This surface is more

or less extensive in accordance with

the character of the lingual lobe. In

most instances the measurement from

Buccal the summit of the cusp or ridge to the

Ridge . to

cervical line is about one-half that of
the same measurements on the buccal
surface. From mesial to distal a well-
rounded convexity is present, while
from the occlusal margin to the
cervical line it is straight or only
slightly convex. The surface passes
so gradually into the mesial and distal
surfaces that no definite lateral mar-
gins exist.

The Mesial Surface of the
Crown (Fig. 124). — In the region of
the occlusal margin this surface is prominent, with a marked convexity
from buccal to lingual; but as the cervical margin is approached, the
surface recedes to the distal, and is
flattened or is' possessed of a slight
general convexity. The occlusal and
cervical margins alone are well de-
fined, the buccal being gracefully
rounded, while the surface passes to
the lingual with a long curve.

The Distal Surface of the
Crown (Fig. 125). — There is but
little difference between this and the
mesial surface; the occlusal portion of
the surface is somewhat less promi-
nent, resulting in less of the bell-
shaped appearance to this side of the
crown.

The Neck of the Tooth.— The
neck of this tooth is marked by a well-defined constriction, the enamel
of the crown suddenly folding in to meet the cementum of the root at
the cervical line, forming a band or ridge which completely encircles the

Triangular Ridge

Distal Pit

Lingual Ridge

Cervical
Ridge

FlG. 125. — Right Lower First Bi-
cuspid, Distal Surface.

THE LOWER BICUSPIDS.

I»3

tooth. The amount of constriction appears to be evenly distributed
between the various parts, so that, viewed in all directions, the neck
becomes a distinctive feature of the tooth.

The Root of the Lower First Bicuspid. — The root of this tooth
is usually straight and tapers gradually from base to apex. In rare
instances it is bifurcated, and when thus formed, those portions beyond
the point of separation are more or less crooked. In the single root the
apical third often curves slightly to the distal. The buccal and lingual
sides are convex throughout their entire length, while the mesial and
distal may be slightly convex, flattened, or provided with a slight longi-
tudinal concavity. In passing from buccal to lingual the mesial and
distal sides converge, thus resulting in a narrowing of the lingual side
of the root.

Buccal Cusp

LOWER SECOND BICUSPID.

Calcification Begins, Between the Fourth and Fifth Year.
Calcification Completed, Eleventh to Twelfth Year.
Erupts, Eleventh to Twelfth Year.
Average Length of Crown, .31.

Average Length of Root, .56.

Average Length over All, .87.

In general contour this tooth is similar to the lower first bisuspid,
excepting that the crown is somewhat more rounded and the lingual
cusp more fully developed, this latter feature causing it to closely resemble
the upper bicuspids. The crown is frequently a trifle shorter than that
of the lower first bicuspid, but the length
of the root generally exceeds that of the
latter, making this the longer tooth of
the two.

The Occlusal Surface of the
Crown (Fig. 126). — The occlusal sur-
face of this tooth presents a greater
variety in form than any other tooth of
its class. The general outline of the
surface is that of a broken circle, in
most instances the mesial and distal
margins showing almost as much of a
convexity as that of the buccal and lingual. The summit of the
buccal cusp usually extends well toward the center of the surface,
but it is sometimes forced toward the buccal by an increased develop-
ment in the lingual cusp. The buccal grooves, which cross the mesial

Triangular
Ridge

Distal Pit

Buccal
Groove

^usp

Fig. 126

Lingual Ci:
— Lower Second Bicuspid,
Occlusal Surface.

1 84

ANATOMY.

Buccal Ridge

Distobuccal
Angle
Cervical
Ridge

Buccal
Groove

Cervical
Line

FlG. 127.-
cuspu

and distal cutting-edges of the buccal cusp, are seldom so well defined
as those of the first bicuspid, but they occasionally pass over these

marginal ridges and form well-marked
grooves, which end in the mesial and
distal pits. The triangular ridge of the
buccal cusp is usually more prominent
than in the first bicuspid, and divides
the surface into two portions, which are
about equal in extent, the center of each
portion being provided with a small pit
— the mesial and distal pits. As in the
first bicuspid, the mesio- and disto-
marginal ridges begin at the mesial and
distal angles of the crown, pass to the
lingual, and, uniting, form the lingual
ridge or cusp. The lingual cusp, while

Right Lower Second Hi- ,. ,, , , , .

(1 Buccal Surface generally well developed, is never so

prominent as the buccal. The lingual
lobe is sometimes divided by a groove which passes from buccal to
lingual, thus forming three cusps upon the surface. When this latter
condition is present, the mesial and distal grooves are fully outlined
from the mesio- and disto-marginal Buccal Grooves

ridges to the center of the surface, where
they unite with the groove previously
referred to and form a central pit or
fossa. Another form frequently met
with is one in which the surface closely
resembles that of the upper bicuspids,
two well-defined cusps being present,
separated from each other by a central
groove, which passes from mesial to
distal and joins the triangular grooves
at these joints. The resemblance to the
upper bicuspids is further increased by
the presence of two small pits, one on
the mesial and one on the distal half of
the surface.

The Buccal Surface of the Crown

Distomar-
ginal Ridge

Summit
of Lin-
gual Cusp

Fig 128. — Lower Second Bicuspid,
Lingual Surface.

(Fig. 127). — The principal

variation between this and the buccal surface of the lower first bicuspid
is that it is less extensive and the buccal grooves somewhat less defined.

THE LOWER BICUSPIDS.

185

Lingual Cusp

Mesiomar-
ginal Ridge

Triangular Ridge

Buccal Cusp
Buccal Ridge

Fig. 129.

—Lower Second Bicuspid,
Mesial Surface.

It presents a general convexity, which is most pronounced near the
center, between which point and the occlusal margins it is slightly inclined
to flatness. The summit of the buccal cusp is usually to the mesial of
the center of the occlusal mar-
gin, so that the mesial cutting-
edge is considerably longer than
the distal, this fact also resulting
in forcing the buccal ridge to the
mesial of the center of the sur-
face. The mesial angle of the
crown, as observed when looking
directly upon the buccal surface,
is in a direct line with the mesial
side of the root, while the distal
angle extends beyond this cor-
responding line, and gives a
prominent or bulging appearance
to this section of the crown.

The Lingual Surface of the Crown (Fig. 128).— Proportionately,
this surface is more extensive than the corresponding surface of the first
bicuspid, this increase being produced by the additional development
of the lingual cusp. It is well rounded from mesial to distal, and passes

into these surfaces without the existence
of a positive line of separation. From
the cervical line to the occlusal margin a
slight convexity is present. The general
outline of the surface is much influenced
by the conditions present upon the oc-
clusal surface.

The Mesial Surface of the Crown
(Fig. 129). — In the region of the occlusal
margins this surface is decidedly convex
from buccal to lingual, but in passing
toward the cervical line a gradual flat-
ness is apparent, which, however, seldom
amounts to a perfect plane. While
there is a gradual convergence of this
and the distal surface toward the root, it is not so marked as that of
the first bicuspid, resulting in less of the bell-shaped appearance to
the crown.

Buccal Groove

Lingual
Cusp

Buccal
Cusp

FlG. 130.

-Lower Second Bicuspid,
Distal Surface.

1 86 ANATOMY.

The Distal Surface of the Crown (Fig. 130). — The description
given of the mesial surface applies equally well to this, there being but
slight variation existing between the two. Occasionally this surface
will present a greater convexity in the region of the occlusal margin, but
this is not a constant feature.

The Neck of the Tooth. — The crown of the tooth being some
what smaller, and the root proportionately larger and longer than that
of the first bicuspid, results in diminishing the amount of constriction
at the neck, and for that reason this feature is less definite.

The Root of the Tooth. — As previously stated, the root of this
tooth is larger and longer than that of the first bicuspid. The mesial
and distal sides are flattened and frequently provided with a longitudinal
groove. In some instances it is rather blunt, ending in a heavy, rounded
apex; in others it tapers very gradually from the base to the apex, ending
in a slim, pointed extremity.

THE LOWER MOLARS.

THE LOWER FIRST MOLAR.

Calcification Begins, about One Month before Birth.
Calcification Completed, Ninth ro Tenth Year.
Erupts, Sixth to Seventh Year.
Average Length of Crown, .30.

Average Length of Root, .52.

Average Length over All, .82.

The process of development in this tooth corresponds to that of
the upper first molar, calcification beginning upon the various cusps
as early as the eighth fetal month, the crown being completely calcified
by the fifth year, the roots formed and the root apices established by
the eleventh year. There is, however, one important difference between
the development of this tooth and the corresponding upper tooth : that
of calcification taking place usually horn five centers instead of four, and,
as a result, we find the occlusal surface provided with five well-developed
cusps separated from one another by five developmental grooves. When
compared with the upper first molar, the crown of this tooth is found to
be somewhat less in size; in general outline it is subject to a greater varia-
tion and is much more angular in its nature. The mediodistal measure-
ment of the crown is nearly always greater than the buccolingual, and
the length of the crown from the occlusal margins to the cervical line is
proportionately less than that of the corresponding upper molar.

The Occlusal Surface of the Crown (Fig. 131). — The general
outline of the crown is best studied when looking directly upon this

THE LOWER MOLARS. 187

surface. Two principal varieties exist: one in which the sides or mar-
gins of the surface appear to be flattened or straightened out, and the
other when these same margins are gracefully rounded. In either
form the buccal line is the longest, so that the mesial and distal lines
converge to meet the lingual. This common form gives to the buccal
angles an acute character, while the lingual angles are about equally
obtuse. The surface is divided into five distinct or developmental
portions, each of which is surmounted by a cusp, named, as their location
indicates, mesiobuccal, buccal, distobuccal, mesiolingual, and disto-
lingual. Separating these parts are Bu c } Bucc .

five developmental grooves — the Cusp Groove

mesial, the distal, the buccal, the

p Mesiobuc

lingual, and the distobuccal. The D c ^cu"p 8 i calCusp

four former cross the marginal Distal § I central

ridges from the various surfaces Gr "" vt ■ |

B J ■ Mesial

and end in the central fossa, while Distal Pit j | Groove

the latter passes from the disto-
buccal angle and joins the distal

Disto- Lingual Mesio-

groove, their union being marked Ungual Groove Ungual

° Cusp Cusp

by a slight depression Or pit — the Fig. 131.— Lower First Molar, Occlusal

distal pit. Branching off from the Surface.

various grooves are a number of supplemental grooves, the presence of

which results in the production of a number of smaller ridges.

The Marginal Ridges of the Occlusal Surface. — Properly speaking
these are only two in number, the mesiomarginal ridge and the disto-
marginal ridge. Those margins which correspond to the buccal and
lingual ridges of the upper molars are so broken by the various cusps
and developmental grooves that a definite marginal ridge scarcely exists,
as will be observed by the description of these parts.

The mesiomarginal ridge is strongly outlined, passing from the
mesiobuccal to the mesiolingual angle of the crown in the form of a bold
angular ridge. In some instances it is broken near the center by the
mesial groove passing over it to reach the mesial surface, in others being
further divided by numerous small supplemental grooves.

The distomarginal ridge is much shorter and less decided than the
mesial, and extends from the distobuccal to the distolingual angle.
In nearly every instance it is broken by the distal groove, which crosses
it to reach the distal surface.

The buccomarginal ridge is formed by the various ridges which
descend in a mesial or distal direction from the three buccal cusps.

1 88 ANATOMY.

Near the center the margin is broken by the buccal groove, and it is again
broken at its distal third by the distobuccal groove, both of which pass
over it to reach the buccal surface of the crown. This is much the
longest margin of the surface, and in its entirety presents a gradual
buccal convexity.

The linguomarginal ridge is principally made up of the distal incline
from the mesial cusp, and by the mesial incline from the distal cusp.
Near the center it is broken by the lingual groove, which passes over it
to reach the lingual surface. This margin, unlike the buccal, is not
always convex, but in many instances is almost a straight line, extending
from the mesial to the distal angle.

The Cusps (Fig. 131). — The Mesiobuccal Cusp (Fig. 131). — This
is usually the largest, though not always the longest, cusp of the group.
It is bounded by the mesial and buccal surfaces and by the mesial and
buccal grooves, which together form the mesiobuccal triangular groove.
Descending from the summit of this cusp to the distal is a well-defined
ridge — a part of the buccomarginal ridge — while in a mesial and lingual
direction the descending ridges contribute to both the bucco- and mesio-
marginal ridges. Descending toward the center of the surface and
ending in the central fossa is the mesiobuccal triangular ridge.

The Buccal Cusp (Fig. 131). — This cusp, which is placed a little
to the distal of the center of the buccal surface, is separated from the
mesiobuccal cusp by the buccal groove, and from the distobuccal cusp
by the distobuccal groove. It is about one-half the size of the mesio-
buccal cusp, and a trifle less in length. Descending from it are two ridges,
one in a mesial and one in a distal direction, which form a portion of the
buccomarginal ridge; descending to the buccal surface is the buccal
ridge, while the central incline gives place to a fourth ridge — the bucco-
triangular ridge.

The Distobuccal Cusp (Fig. 131). — This cusp is much the smallest
of the five, and is located at the distobuccal portion of the crown, in
some instances being nearest the buccal surface, in others forced to the
distal by an increase in the size of the buccal cusp. It is separated from
the buccal cusp by the distobuccal groove, and from the distolingual cusp
by the distal groove. The ridges which descend from it contribute to
both the bucco- and disto-marginal ridges, and descending toward the
distal pit is the distobuccal triangular ridge.

The Mesiolingual Cusp (Fig. 131). — This cusp is second in size,
and frequently the longest and most pointed. It has for its boundaries
the mesial and lingual surfaces, and the mesial and lingual grooves.

THE LOWER MOLARS.

189

The ridge which descends from it in a mesiobuccal direction assists in
forming the mesiomarginal ridge, while that which passes to the distal
forms a part of the linguomarginal ridge. In the direction of the central
fossa a pronounced ridge is present — the mesiolingual triangular ridge
— which is often supplemented by one or more smaller ridges running in
the same direction.

The Distolingual Cusp (Fig. 131). — This cusp usually occupies the
distolingual portion of the crown, although sometimes being forced well
toward the lingual by the

Buccal Groove

Distobuccal
Cusp

Buccal Pit

Mesioling-
ual Cusp

Mesiobuc-
cal Cusp

Fig. 132 — Lower First Molar, Buccal
Surface.

distobuccal cusp. It is
separated from the mesio-
lingual cusp by the lingual
groove, and from the dis-
tobuccal by the distal
groove. Two of the ridges
which descend from it as-
sist in forming the linguo-
and disto-marginal ridges,
while the one which de-
scends the central incline is
the distolingual triangular
ridge. The central incline
of the mesiobuccal, buccal,
mesiolingual, and disto-
lingual cusps contribute to
the formation of the central fossa, while the buccal, distobuccal, and
distolingual central inclines assist in forming the distal pit or fossa.

The Buccal Surface of the Crown (Fig. 132). — This is the most
extensive of the lateral surfaces of the crown. It is convex from mesial
to distal, and also from the occlusal margin to the cervical line. The
width of the crown from the mesial to the distal angle is always somewhat
greater than that at the cervical line, the difference being governed by
the typal form of the tooth. A little to the mesial of the center of the
surface is the buccal groove, which, after crossing the buccomarginal
ridge, is usually quite deep; but as it proceeds in the direction of the
root it gradually disappears, or it may end abruptly in a well-defined pit
— the buccal pit. The distobuccal groove enters the surface near the
distobuccal angle, and gradually becomes less pronounced as it passes
rootward. It is seldom so well defined as the buccal groove, and usually
ends when about half-way to the cervical line. The occlusal margin

190

ANATOMY.

Distolingual
Cusp

Mesiolingual
Cusp

Lingual
Groove

is made irregular by the presence of the three buccal cusps; the cervical
margin is nearly straight from mesial to distal, and is surmounted through-
out by a strong enamel fold, the cervicobuccal ridge. The mesial margin

is longer than the distal, but neither of
them is well defined.

The Lingual Surface of the Crown
(Fig. 133). — This surface is smooth and
convex in every direction. It is gener-
ally divided into two portions, a mesial
and a distal, which are nearly equal in
extent. This separation is formed by
the lingual groove, which is sometimes
deep and sulcate, at others shallow, and
not infrequently entirely wanting. The
surface is nearly one-third less in extent
than the buccal, the convergence of the
mesial and distal surfaces in passing to
the lingual accounting for this difference.
The occlusal margin is formed by the
double incline of the two lingual cusps; the cervical margin is either
straight or slightly concave in the direction of the occlusal surface,
while the mesial and distal margins are rounded and poorly defined.

The Mesial Surface of the Crown (Fig. 134). — This surface is
inclined to flatness, with a slight
bulging near the center, which marks
the point of contact with the approxi-
mate tooth. It is usually smooth, and
unbroken by developmental or other
grooves, although the mesial groove
occasionally traverses it after crossing
the marginal ridge from the occlusal
surface. Near the center of the cer-
vical third a slight concavity is often
present. The margins of the surface
are somewhat irregular, the occlusal

Fig.

133. — Lower First Molar,
Lingual Surface.

Central Fossa

Mesiobuc-
cal Cusp
Cervico-
buccal
Ridge

Mesiolin-
gual Cusp

Fig.

134. — Lower First Molar, Mesial
Surface.

margin being made irregularly concave

by the ridges which descend from the

two mesial cusps; the cervical margin is slightly concave in the direction of

the occlusal surface, and, while the buccal margin inclines to the lingual as

the occlusal surface is approached, the lingual is almost perpendicular.

THE LOWER MOLARS.

I 9 I

The Distal Surface of the Crown (Fig. 135). — Unlike the mesial,
this surface is possessed of a decided convexity in every direction. It
is surmounted by a portion of the distobuccal and distolingual cusps,
and is frequently broken by the distal groove, which reaches it after
crossing the marginal ridge from the occlusal surface. The occlusal
margin is irregularly formed of the marginal ridges which descend from
the distobuccal and distolingual
cusps; the cervical margin is
usually straight, while the buccal

Distobuccal

and lingual are rounded and in- cusp

Mesiolin-
gual Cusp

Distolin-
gual Cusp

Fig. 135. — Lower First Molar, Distal
Surface.

definite.

The Neck of the Tooth.—

One characteristic feature of
this tooth is the greater circum-
ference of the crown at the oc-
clusal margin over that at the
cervical line, giving a flaring
appearance to the crown, and
resulting in the production of a
neck which is much constricted.
This is particularly noticeable
when looking upon the buccal surface of the tooth; but when looking
upon the mesial or the distal surface, this feature is not so pronounced,
although the rather heavy fold of enamel which surmounts the cervical
line contributes much to the formation of the neck from these aspects.

The Roots of the Tooth. — The roots of this tooth are two in
number — one of which is placed beneath the mesial, and the other
beneath the distal half of the crown — and are named the mesial root and
the distal root. The fact that the point of bifurcation is constantly in
close proximity to the neck or crown of the tooth is a sufficient reason
for the statement that two roots exist, rather than a single root with two
branches. The roots are both much flattened from mesial to distal,
and broad at the base from buccal to lingual.

The mesial root is usually the larger and longer of the two. After
leaving its base it generally inclines to the mesial, but beyond the center
of its length it is provided with a distal turn, which in some instances
amounts to a decided crook. The center of the mesial side is occupied
by a longitudinal depression, as is also the distal side, making this part
of the root thin, giving the appearance of an effort to bifurcate, which
condition is occasionally present. The buccal and lingual sides of the

192

ANATOMY.

root are rounded and smooth, and taper gradually to the apex, which
is somewhat broadened from buccal to lingual.

The distal root is usually straight, with a more gradual taper through-
out, ending in an apical extremity more pointed than that of the mesial
root. A longitudinal depression is also present upon both the mesial
and distal sides, but is never so pronounced as that upon the mesial root.
The buccal and lingual sides are convex and smooth. The root possesses
little or no inclination to bifurcate.

Central
Fossa

Linguo-
marginal

Ridge
Mesiolin-
gual Cusp

LOWER SECOND MOLAR.

Calcification Begins, about the Fifth Year.

Calcification' Completed, Sixteenth to Seventeenth Year.
Erupts, Twelfth to Sixteenth Year.
Average Length of Crown, .27.

Average Length of Root, .50.

Average Length over All, .78

This molar differs in so many particulars from the lower. first molar
that a separate description will be called for. The principal variation
is usually found in the absence of the fifth lobe or cusp,* resulting in the
production of an occlusal surface much less complicated.

The Occlusal Surface of the Crown (Fig. 136). — When the crown
is studied by looking directly upon this surface, the variations between

this and the first molar are
readily noted. Four equally
proportioned cusps are observed,
separated from each other by
Distobuccai four developmental grooves. A
single pit or fossa is present, the
four grooves arising from this
one point. In general outline
two principal varieties exist: one
in which the opposite sides of
the crown are nearly of the same
length, and parallel with each
other, with the angles rounded;
the other, in which either the
buccal or lingual margin is the longest, with the mesial and distal
margins converging one way or the other, as the case may be. The
marginal ridges are formed in a manner similar to those of the first molar,
with the exception of the distal portion of the buccal ridge, which is not

*When five cusps are present, the anatomy of this surface does not differ from that of the
first molar.

Bucco-
marginal
Ridge
Mesiobuc-
cal Cusp

SO ~,~

-Lower Second Molar, Occlusal
Surface.

THE LOWER MOLARS.

193

Distobuccal Buccal Mesiobuccal
Cusp Groove Cusp

Distal
Surface

Mesial
Surface

Fig 137. — Lower Second Molar,
Buccal Surface.

broken by a developmental groove. Each marginal ridge is divided
near its center by one of the grooves of development, the mesial groove
crossing the mesiomarginal ridge, the buccal groove crossing the bucco-
marginal ridge, the lingual groove
crossing the lingual ridge, and the
distal groove passing over the distal
ridge. In many instances numerous
supplemental grooves are present,
which in turn form a number of
smaller ridges. The four cusps are
the mesiobuccal, distobuccal, mesio-
lingual, and distolingual. In a gen-
eral way they are similar to the cusps
of the first lower molar, excepting
that they are somewhat larger and
probably less pointed and less angu-
lar. Each cusp is provided with a
number of ridges, which descend from
the summit to the base, two of these contributing to the formation of
the marginal ridges, one passing to the buccal or lingual, and one, the

triangular ridge, descending the cen-
tral incline of each cusp. The names
given to these various ridges are iden-
tical with those of the first molar.

The Buccal Surface of the
Crown (Fig. 137).— The principal
difference between this and the cor-
responding surface of the first molar
is that produced by the absence of the
fifth cusp, the surface being divided
into two parts instead of three. The
single division is caused by the buccal
groove, which reaches the surface after
crossing the buccomarginal ridge
from the occlusal surface. The posi-
tion of this groove is usually a little
to the mesial of the center of the
surface. Like the buccal groove of the first molar, it may disappear
gradually as it passes toward the cervical line, or it may end in a well-
marked pit — the buccal pit.

I 3

Lingual
Groove

Fig 138. — Lower Second
Molar, Lingual Surface

194

ANATOMY.

The Lingual Surface of the Crown (Fig. 138).— This surface
so closely resembles the corresponding surface of the first molar that
it is somewhat difficult to distinguish one from the other. The occlusal

~- 2 3
.2(3 .2 .2<3

Mesiobuc-
cal Angle

Lingual
Grove

Fig, [39. Lower Second Molar, Mesial Surface.

margin may be a trifle less irregular, and in some instances more ex-
tensive, than the buccal surface, this latter feature seldom occurring in
the first molar.

Mesial Groi

Fig. 140. — Lower Second Molar, Distal Surfac e.

The Mesial Surface of the Crown (Fig. 139). — This surface cor-
responds to the mesial surface of the first molar, being flattened or
slightly convex from buccal to lingual, with an inclination to a slight
depression or concavity near the cervical margin.

THE LOWER MOLARS.

195

The Distal Surface of the Crown (Fig. 140). — On account of
the absence of the fifth cusp, this surface is less complex than that of
the first molar. It is convex in all directions; in most instances smooth,
in others broken by the distal groove, which reaches it after crossing
the distomarginal ridge from the occlusal surface.

The Roots of the Tooth. — Like the first molar, these are two in
number, a mesial and a distal. They are much less constant in form,
are often nearer together, and in some instances united. When the
two roots exist — which may be considered the normal condition — they
are less flattened upon their mesial and distal sides, with the longitudinal
depression wanting or but slightly apparent. These roots, therefore,
are more rounded in general, taper more gradually from neck to apex,
and end in a rounded apex, this often being provided with a slight distal
curve.

LOWER THIRD MOLAR.

Calcification Begins, Eighth to Ninth Year.

Calcification Completed, Eighteenth Year.
Erupts, Sixteenth to Twentieth Ve\r.
Average Length of Crown. .26.

Average Length of Roots, .36.

Average Length over All, .62.

This tooth is probably subject to a greater variety in form than
any other. There are, however, two varieties which are most frequently
met with. In one the crown of the
tooth is similar to the lower second
molar, being provided with four
cusps, which are separated from one
another by four developmental
grooves (Fig. 141). The other is
similar to the lower first molar,
having five cusps and five develop-

mental grooves.

Mesio-
mar-
ginal

Ridge

While these two forms are those
most commonly met with, the oc-
clusal surface may be so broken by
numerous supplemental and devel-

that even six or Fig.

well-defined cusps may be
present. Whatever complications may exist upon the occlusal surface,
a central fossa is usually present, from which radiate the various develop-

opmental grooves
eight

rao

141. — Lower Third Molar,
Occlusal Surface.

196

ANATOMY.

mental grooves. When the central fossa is absent, the space which it
should occupy is usually taken up by a rounded cusp, by the interference
of which the grooves are prevented from uniting, and their course is
much distorted. Along with these variations, the tooth is subject to
much variety in size. In some instances the crown is one-third less
in circumference than that of either the first or second lower molars,
while in others it is a trifle greater. The increase in the size of the crown
is generally accompanied by an increase in the number of cusps. One
feature very common to the crown is its inclination to the circular form,

Disto-
lingual
Cusp

Disto- Mesio-

buccal buccal

Cusp Cusp

Buccal
Groove

Fig. 142. — Lower Third Molar,
Buccal Surface.

FlG. M.v~ Lower Third Molar,
Lingual Surface.

almost resulting in the absence of the angles common to molars in general.
The marginal ridges are, of course, subject to the ever-varying con-
ditions to be found upon the occlusal surface; in general, they are poorly
defined, and are frequently crossed by numerous small supplemental
grooves, dividing them into many minute tubercles. The latter are
smooth and strongly convex, with their general outlines much influenced
by the number of cusps.

The Roots of the Tooth.— While this tooth is strongly inclined
to be two-rooted, like the other lower molars, this condition is by no
means the common one. Like the crown, the roots are probably more
variable than those of any other tooth. A single conic root may be

THE LOWER MOLARS.

I97

present, or a mesial and a distal root may exist; again, the mesial root
may bifurcate, thus resulting in three. In some instances, four, or even
five, branches may be given off from a common base. When more

Fig. 144. — Types of Lower Third Molars.

than two roots are present, they are usually much twisted or crooked,
and, while generally inclined to the distal, are liable to branch in various
directions.

A better idea in the variations in this tooth may be had from the
accompanying illustration (Fig. 144).

CHAPTER X.
The Pulp-cavities of the Teeth.

In the preceding chapters the study of the teeth has been confined
to their external forms; it will now be necessary to learn something of
their internal anatomy, and for this purpose various dissections of each
individual tooth must be made.

Dissections. — First, a longitudinal dissection of each tooth should
be made by sawing or filing from labial to lingual in the anterior teeth,
of from buccal to lingual in the posterior teeth. Second, a longitudinal
dissection by sawing from mesial to distal. Third, numerous transverse
dissections by sawing through the crown or root at various points.

These dissections will expose to view a central cavity with outlines
closely corresponding to those of the tooth itself. This is called the
pulp-cavity, and in the vital tooth contains the formative and life-sustain-
ing substance of th dentin, the dental-pulp. The pulp-cavity is divided
into two principal parts, that portion within the crown of the tooth being
the pulp-clnniihi r, while that traversing the root is the pulp-canal. At
the apex of the root the canal ends in a small foramen, the apical fora-
men,* which transmits the blood-vessels and nerves to the pulp. The
pulp-chamber occupies the center of the crown and is always a single
cavity; the pulp-canals are prolongations from this central cavity, and
are usually one for each root, although in some instances two or more
canals are present in a single root. The form of the pulp-chamber
varies with the shape of the crown, the outline of the cutting-edge in the
incisor teeth being reproduced in that part of the chamber nearest to the
cutting-edge, while in the bicuspids and molars the occlusal surface is
reproduced on the wall of the pulp-chamber, immediately beneath it.
the lateral walls corresponding to the various sides of the tooth. In
the incisors and cuspids the pulp-chamber passes so gradually into the
pulp-canal that a positive line of demarcation between the two is not
observed. In the bicuspids and molars the canals may be readily
distinguished by a sudden constriction and branching out of the cavity
into the vairous roots, which prolongations gradually decrease in size

*In many instances there is more than one foramen.

198

THE PULP-CAVITIES OF THE TEETH. IQQ

until the apical foramen is reached. The size of the pulp-cavity is much
influenced by the age of the tooth, its functional activity, character of
the occlusion, etc. The tooth-pulp, as the formative organ of the dentin,
gradually decreases in size as the tooth develops (see Development of
the Teeth), and as a result of this action the youngest teeth are provided
with the largest pulp-cavities. At the time of eruption of a tooth, the
diameter of the pulp-cavity is about equal to one-half the diameter of
the crown, while the length of the canal must, of necessity, accord with
the extent of root-calcification. As the growth of the tooth proceeds,
the diameter of both the chamber and canal is gradually diminished;
this gradual reduction in size is continued during the life of the tooth,
and if permitted to proceed until old age, the chamber and canal may
become almost or entirely obliterated. It must be remembered that
while the diameter of the root-canal is diminished with the growth of
the tooth, its length increases, continuing to do so until the time of
complete root-calcification. During the period of root-development the
diameter of the root-canal is greatest at the free or apical end of the root,
at which point it presents a funnel-shaped opening (Fig. 141). As the
root continues to calcify, this funnel-shaped extremity of the canal
advances in the direction of calcification, and finally, as the formative
process nears completion, the mouth of the funnel gradually disappears,
and the apical foramen is established. The various lobes of the teeth
are penetrated by a prolongation of the pulp-cavity, these being called
the horns of the pulp- chamber. The depth to which the horn penetrates
the lobe varies in accordance with the form of the latter. If the tooth
is one provided with long, penetrating cusps, the horns of the pulp-
chamber will also be long, but if the cusps be poorly formed, the horns
of the chamber will be short. In the anterior teeth, when the lobal
construction is outlined by well-marked developmental grooves, the
horns of the pulp-chamber will be three in number and directed toward
the cutting-edge. These are most marked in young teeth, and gradually
disappear as age advances. The functional activity of the teeth also
serves to materially reduce the size of the pulp-chamber. Thus, when
opposing teeth occlude squarely and firmly against each other, with more
or less -rubbing or sliding during mastication, the external surface is
prone to rapid abrasion, and, as a direct result of this external change,
the pulp-chamber undergoes a corresponding alteration by^a growth
of secondary dentin about its walls.

200 ANATOMY.

THE PULP-CAVITIES OF THE UPPER TEETH.

Upper Central Incisor. — Figure 145 represents a number of
labiolingual sections presenting the relative size and shape of the pulp-
cavity in the upper central incisor at various ages. In No. 1 the con-
dition existing at about the sixth year, or at a time immediately prior
to the eruption of the tooth, is shown. The tooth-crown is fully formed
and calcified; the cervical line may be observed, as well as a small portion
of the root-wall. The pulp-chamber, which is represented by the dark

Fig. 145. — The Pulp-cavity in the Upper Central Incisor, from the Sixth to

the Tenth Year.

portion of the cut, occupies about one-third of the diameter of the crown
at its greatest width. The pulp-chamber at this age, when viewed
in this direction, forms almost a perfect cone, the base of which is directed
upward or toward the future extremity of the root, and its apex down-
ward in the direction of the cutting-edge of the tooth. The apex of the
cone may end somewhat abruptly, or it may be lengthened into a slender,
horn-like projection, extending well toward the cutting-edge. No. 2
represents the same tooth about the seventh year, or at a time shortly
after its eruption. The pulp-chamber has become slightly reduced in
its basal diameter, while but little change has taken place in the apex.
That portion of the pulp-cavity above the cervical line represents a part
of the future pulp-canal. At this age the canal is a direct continuation
of the conic pulp-chamber, ending above in a broad, funnel-shaped
extremity. No. 3 shows the condition of the cavity about the eighth year.
The diameter of the pulp-chamber is considerably diminished, the apex
has slightly receded, and the horn-like projection has partly disappeared.
The increase in the length of the canal is about 3/16 of an inch over its
length at seven years. The two parallel sides of the canal have lengthened

THE PULP-CAVITIES OF THE UPPER TEETH. 201

proportionately, and the funnel-shaped extremity is reduced in diameter
owing to the gradual narrowing of the roots-wall. No. 4 gives the
relative size of the pulp-chamber and canal at the ninth year, or at a
time when root-calcification is nearing completion. The decrease in
the capacity of the chamber is readily apparent; the horn-like projection
has disappeared and the parallel sides of the canal are partly extended
into the chamber, thus reducing the length of the cone. In the canal a
greater reduction has taken place in its diameter, while its length has
increased about 1/4 of an inch over that at eight years, and the diameter
of the funnel-shaped opening is but little greater than that of the body

Fig. 146. — -Pulp-cavity in the Upper Central Incisor.

of the canal. No. 5, which represents a section of the tooth about the tenth
year, shows calcification in the root completed, and the apical foramen
established. A glance at the illustration will show the gradual decrease
in the capacity of the pulp-cavity and the completion of its growth in an
apical direction. At this stage of development the fan-shaped extremity
of the canal gradually disappears, and for the first time in the life of the
tooth the canal partakes of the external root form throughout its entire
extent.

Figure 146, A, represents the size and form of the average pulp-
cavity in the adult upper central incisor. In its entirety it represents a
double cone, with a common base near the cervical line, the pulp-cavity
forming one cone and the pulp-canal the other. At this common base
the cavity assumes its largest diameter, which measurement is approxi-
mately equal to one-fourth the labiolingual diameter of the tooth. The
extent and form of the lower cone, or that represented by the pulp-
chamber, varies in the adult tooth with the tooth type. Thus, in the
nervous type the cone is long and narrow, with the apex ending in a hair-

202 ANATOMY.

like projection. In the tooth of the lymphatic temperament the cone
is prone to be wide, with its apex ending abruptly. In the sanguine and
bilious types the form and extent of the cone do not partake of either
of the foregoing extremes, but, in keeping with the outline of the crowns.
are intermediate between them. Figure 146, B, represents the average
condition of the pulp-cavity in the central incisor in advanced age, and
shows a general reduction in the size of both chamber and canal. A
further study of the pulp-chamber and canal may be made by a mesio-
distal section made through the long axis of the tooth (Fig. 146, C).
The outline of the cavity, viewed in this way, closely follows the outline
of the crown and root of the tooth. There is no distinct division between
the chamber and the canal, the former gradually blending into the latter.
The outline of the entire cavity is that of a single cone, with its base
directed toward the cutting-edge and its apex in the direction of the
apical extremity of the root. The lower margin of the pulp-chamber,
or that nearest the cutting-edge of the crown, is broad from mesial to
distal and thin from labial to lingual. This margin in the average adult
tooth is about on a line with the center of the labial surface of the crown,
and the lateral walls of the cavity as they pass upward converge slightly.
and finally blend into the walls of the canal at a point somewhat beyond

Fig 147. rransverse Sections, Root of Upper Central Incisors, Slightly

Enlarged

the cervical line. During the early life of the tooth the margin of the
chamber nearest the cutting-edge presents three well-defined horns,
corresponding to the three rudimentary lobes found upon the cutting-
edge at this period. These horns rapidly disappear, and are seldom
found after the fifteenth year. In certain tooth types, however, the
mesial and distal horns may continue ] 'resent until adult age, and even
into middle life, but when this occurs it is not the result of the temporary
tooth form, but is occasioned by the permanent angular outline of the
crown.

Figure 147 represents a number of transverse sections of an upper
central incisor, showing the outline and relative size of the pulp-cavity

THE PULP-CAVITIES OF THE UPPER TEETH. 203

in passing from the base of the crown toward the apex of the root. No. i
shows the outline of the cavity at the cervical line; No. 2 represents the
condition 1/8 of an inch nearer the apex of the root; No. 3 is from the
center of the root length, while No. 4 is from the region of the apex.

Upper Lateral Incisor. — The pulp-cavity in the upper lateral
incisor is so nearly identical with that of the central that it will only be
necessary to call attention to one or two points which are at variance.
Figures 148 shows the five stages as represented by the growth of the
tooth. In general it will be observed that the cavity is much smaller
than that of the central incisor, but this difference is to be accounted for

Fig. 148. — Pulp-cavity in the Upper Lateral Incisor, from the Sixth to the

Tenth Year.

in the smaller proportions of the tooth. No. 1 shows the condition of
the crown and pulp-cavity about the sixth year, the pulp-cavity occupying
a large portion of the partly clacified tooth-crown. No. 2 represents
the conditions present at the seventh year, or about the time of the
eruption of the tooth. The pulp-chamber at this age resembles a perfect
cone, the base of which reaches to the root-walls, and faintly outlines
the beginning of the future pulp-canal. In No. 3, at eight years, the
length of the root has increased about 3/16 of an inch, and the parallel
sides of the walls of the pulp-canal have made their appearance. In No.
4, at nine years, by the growth of the root the canal has considerably
increased in length and at the same time much decreased in diameter,
while in No. 5, at ten years, the root is completely formed, the apical
foramen established, and the maximum size of the entire pulp-cavity
in the fully formed tooth shown.

Figure 149, A, shows the average condition of the pulp-cavity in
the upper lateral incisor at adult age, while figure 149, B, represents
the same tooth in old age. In a mesiodistal section — figure 149, C — a
very close resemblance to the pulp-cavity in the central incisor will be

204 ANATOMY.

noticed. While the pulp-cavity is smaller than that of the central
incisor, it is usually a trifle larger in proportion to the size of the tooth.
Owing to the marked constriction at the neck of this tooth, there is
occasionally found a slight line of distinction between the pulp-chamber
and canal, but in the majority of instances this is not to be observed.
The horns of the pulp-chamber are in every respect similar to those
of the central incisor, excepting when they exist permanently, in which
case the mesial horn is usually the longest. By the transverse sections
shown in figure 149, the gradual decrease in size and change in form in
the root-canal are presented, the sections being similar to those made
in the root of the central incisor.

Fig. 149.

Upper Cuspid. — The pulp-cavity of this tooth is in general similar
to that of the incisors, excepting that the coronal extremity of the chamber
is conic and inclined to a horn-like projection which penetrates the single
cusp of the tooth-crown in the direction of its summit. Figure 150
represents a number of labiopalatal sections. No. 1 shows the condi-
tion of the pulp-cavity about the seventh year, or fully five years before
the eruption of the tooth. The pulp-chamber partakes of the cone shape
previously referred to, but the margins, instead of being straight lines,
are somewhat bowed or concave, thus conforming more closely to the
outline of the crown. The central horn of the chamber is proportionately

THE PULP-CAVITIES OF THE UPPER TEETH. 205

longer than that of the incisors, in correspondence with the cusp of the
tooth. At this age the formative process has barely extended to the
root-walls; therefore, the width of the cavity it about equal to its length.
In No. 2, at eight years, an increase in the capacity of the chamber over
that of the incisors is shown, this being the result of the greater bulk in
the tooth-crown. The cone-like outline of the chamber is somewhat
broken by an effort of its margins to follow the outline of the crown.
In No. 3, at nine years, the principal change has taken place in the
canal, which has lengthened fully 3/16 of an inch, and the funnel-shaped
extremity, instead of joining with the pulp-chamber direct, is continued
below by two parallel walls to the true beginning of this cavity. x\t ten

red

Fig. 150. — Pulp-cavity in the Upper Cuspid, from the Seventh to the Twelfth

Year.

years, No. 4, a more marked transformation has taken place in both
portions of the cavity. The diameter of the chamber at the cervical line
has diminished, as has also the length of the cone. The increase in the
length of the root, which has been proportionately greater than that of
the preceding year, has extended the length of the canal about 3/8 of
an inch. The walls of the canal are no longer parallel with each other,
but are inclined to follow the root-outlines. The funnel-shaped opening
is much reduced both in length and breadth. No. 5 represents the
condition at the time of the eruption of the tooth, or about the twelfth
year. The general outline of the pulp-cavity is that of a double cone,
with a common base at a point nearly corresponding to the cervical line.
The diameter in both the chamber and canal has considerably decreased,
while the central horn in the former has further receded. The calcifica-
tion of the root externally is about complete and the foramen formed.
In this particular the cuspid tooth differs from the incisors, and in fact
from all other teeth, in having its root-calcification about completed and

2o6

ANATOMY.

the apical foramen established at or soon after the time of its eruption.
Figure 151, A, gives an idea of the capacity of the pulp-cavity in the upper
cuspid at maturity, while figure 150, B, shows the condition in advanced
age. Figure 151, C, is a mediodistal section of a matured upper cuspid.
The coronal extremity of the pulp-chamber is scpaare, and but little
inclined to follow the outline of the mesial and distal cutting-eds^.
The chamber passes into the canal without a mark of separation, and
the latter gradually diminishes in diameter as the apex of the root is

Fig. ici.

approached. At its point of beginning the canal is sometimes inclined
to flatness from mesial to distal, but in passing toward the apex this
tendency disappears, and it becomes more circular in outline. In figure
151, D, a transverse section through the tooth at the cervical line is
shown, giving an idea of the proportionate size and form of the canal in
the adult tooth, while E, F, and G represent transverse sections through
the root of the same tooth at various points between the cervical line
and the apex of the root.

Upper First Bicuspid. — The study of the pulp-cavity in this
tooth differs in many particulars from that of the incisors and cuspids.
First, the line of distinction between pulp-chamber and the root-canal

THE PULP-CAVITIES OF THE UPPER TEETH. 207

or canals is, in most instances, definitely marked by the bifurcation of
the roots and a corresponding branching of the pulp-cavity into two
line canals, one of which occupies the center of each root. This division
of the cavity brings the center of the pulp-chamber almost on a level
with the cervical line. In figure 152, No. 1 shows the partly calcified
crown of the upper first bicuspid at the seventh year. A portion of the
pulp-chamber alone may be studied at this period, and this is found to
be somewhat irregular in outline, with a broadened, funnel-shaped open-
ing above, and two small, cone-like projections below, pointing into
either cusp of the crown. These latter projections are the horns of the
pulp-chamber, and are named in accordance with the cusp which they

Fig 152. — Pulp-cavities of the Upper First Bicuspid, from the Seventh to the

Twelfth Vear

occupy. In very young teeth it is not unusual to find these horns pene-
trating the dentin almost to the enamel-wall. No. 2, at eight years,
shows the crown fully calcified and the outline of the base of the roots
established. The horns of the pulp-chamber have slightly receded, and
the branching of the canals is made manifest by the central deposit of
dentin. In No. 3, at nine years, the capacity of the pulp-chamber is
much decreased, and appears to have receded bodily rootward. The
roots are calcified to about one-third their full length, and the canals
which traverse them are each provided with the funnel-shaped opening
at their free calcifying extremities. In No. 4, at ten years, the decrease
in the size of the pulp-chamber is not only caused by the deposit of
dentin upon the occlusal and lateral walls, but from the direction of the
roots as well. The diameter of the root canals is much less than at nine
years, but the walls are as yet parallel. No. 5 shows the roots fully
formed and the apical foramina established, which condition occurs
about the twelfth year. The horns of the pulp-chamber have receded

208

ANATOMY.

somewhat, and the center of this cavity is now almost on a level with
the cervical line. The canals have assumed the form of the roots them-
selves, and their diameter is much diminished. The illustration shows
the proportionate maximum size of the chamber and canals in this tooth
after completion of surface calcification. It will be observed that the
foramina are proportionately smaller than those of the incisors and
cuspids at a corresponding period, this condition resulting from the
lesser diameter of the roots.

Figure 153, A, illustrates the approximate size and form of the
pulp-chamber and canals at adult age, and attention is called to the

A B C D

F. F G

Fig. 153. — Pulp-cavities of the Upper First Bicuspid, Enlarged about One-third.

appearance of the horns of the pulp-chamber. It will be observed that
the horn which penetrates the buccal cusp is larger and more pointed
than that directed toward the lingual cusp; this condition is fully ex-
plained by the buccal cusp being proportionately larger and longer than
the lingual. In the same figure, B represents the pulp-cavity in the first
upper bicuspid at advanced age. The foregoing description applies
only to the two-rooted bicuspids, but as many of these teeth have but
one root, an additional description will be necessary. When a single
root is present, many varieties in the outline of the pulp-cavity will
be presented; this variation, however, seldom affects the capacity or

THE PULP-CAVITIES OF THE UPPER TEETH. 200.

form of the pulp-chamber. Two distinct canals may exist in the single
root (Fig. 153, C), branching off from the chamber,ione from the buccal and
one from the lingual portion. These canals gradually taper in the direction
of the apex of the root, and may end in a single foramen, or in distinct fora-
mina. Occasionally the canals will unite before reaching the root-apex and
continue as a single canal ending in a single foramen, or they may com-
municate at one point and again diverge and finally end in separate
foramina. In some instances the pulp-canal appears to be a direct
continuation of the pulp-chamber, extending throughout the length of
the root in the form of a flattened canal, with its greatest diameter from
buccal to lingual (Fig. 153, D). When two separate canals exist in the
single root, the outward appearance of the root indicates a near approach
to two roots; when the single flattened canal is present, the root is also
flattened and shows no sign of bifurcation. Reference has been made
to the horns of the pulp-chamber, and in this connection it will be well
to speak of the extent to which they may exist. In that type of tooth
provided with long penetrating cusps the horns will dip well down into
the cusp occasionally to the full depth of the dentin, and in rare instances
may penetrate the enamel. In those teeth lacking in cusp-formation
the length of the horns will be correspondingly reduced, and may be
entirely wanting. In the two-rooted upper first bicuspid the floor of
the pulp-chamber, or that part of the cavity directed rootward, is promi-
nent and rounded in the center, from which point it gradually slopes
toward the entrances to the canals, one of which arises from the extreme
buccal margin, and the other from the extreme lingual margin. Figure
153, E, represents a transverse section of the two roots immediately
below the point of bifurcation. F represents a section of the two roots
midway between the cervical line and the apical extremity, while G is
a transverse section of D at the cervical line.

Upper Second Bicuspid. — The pulp-cavity of this tooth is in
many respects similar to that of the first bicuspid, the principal variations
being in the horns of the chamber, which are proportionately smaller
in correspondence with the diminution in cusp-formation. There is
usually no positive line of demarcation between the chamber and canal,
the latter being quite large, and broad from buccal to lingual. The
extent of the pulp-chamber is sometimes well defined by the presence of
two root-canals, similar to those described in connection with the first
bicuspid. In rare instances the tooth may possess two roots, each of
which would be traversed by a canal. Figure 154 represents the various
stages of the development of the pulp-cavity, as shown by a longitudinal
14

2IO

ANATOMY.

section from buccal to lingual. No. i shows the condition at seven
years, or at a time when a portion of the crown only is calcified, in con-
sequence of which the pulp-chamber alone can be studied at this period.

Fig. 154. — Pulp-cavities in the Upper Second Bit uspid, from the Seventh to

the Twelfth Year.

The buccal and lingual horns of the chamber may be observed penetrat-
ing the dentin in the direction of their respective cusps. No. 2 shows
the advance made in the formative process by the eighth year, or at a

Fig.

155-

A 15

-Section of Upper Second Bicuspid, Slightly Knlarged.

time immediately prior to the eruption of the tooth; the outline of the
chamber is completed and the walls of the future pulp-canal faintly
outlined. At this period there has been but little change in the horns

THE PULP-CAVITIES OF THE UPPER TEETH. 211

of the pulp-cavity. No. 3 shows the condition of the tooth at the ninth
year, or at the beginning of its eruptive period. The diameter of the
chamber has somewhat decreased, the horns have slightly receded, the
funnel-shaped extremity of the cavity has advanced beyond the cervical
line, and is now confined to the canal alone. No. 4 represents the
condition of the pulp-cavity about the tenth year. A gradual decrease
in the diameter of both the chamber and canal is observed, and the horns
of the pulp-cavity are growing less prominent. The length of the root
having increased nearly one-quarter of an inch, we find a corresponding
addition to the length of the canal. No. 5 shows the maximum size of the
pulp-cavity in the upper second bicuspid, which condition accompanies
the completion of the external calcification at the twelfth year. As
previously stated, the cavity, in its entirety, presents no line of separation
between the chamber and canal, but gradually tapers from its broadened
base in the crown to its ending at the apex of the root. In this tooth,
as well as in all those previously described, the apical foramen at the
time of completion of root-calcification is comparatively large, and
readily penetrated during operations upon it. Figure 155 illustrates
a number of sections of an upper second bicuspid at maturity. The
same figure also represents two transverse sections, A being at the cervical
line, B midway between the cervical line and the apex of the root.

PULP-CAVITIES OF THE UPPER MOLARS.

The inner anatomy of the molar teeth being much more complicated
than any of those previously described, it will be found necessary to make
a number of dissections in various directions in order to obtain a com-
prehensive idea of the location and form of the different parts of the
pulp-cavity. The line of demarcation between the pulp-chamber and
canals is always definite, the former occupying a central position in the
crown and seldom extending beyond the cervical line, while the latter
are given off from the floor of the chamber and penetrate the various
roots, their entrances being marked by small funnel-shaped openings in
the floor of the chamber. In the matured tooth the form of the chamber
usually corresponds to that of the crown of the tooth. The lateral walls
of the chamber are four in number, and are named according to their
location — mesial, distal, buccal, and lingual. The average thickness
of these walls at maturity is about equal to the diameter of the pulp-
chamber. In that type of tooth common to the lymphatic temperament
where there is but little constriction at the neck, resulting in the various

212

ANATOMY.

sides of the tooth-crown being nearly parallel with each other, the pulp-
chamber is nearly quadrilateral in form; but in those teeth marked by a
decided constriction at the neck, most marked in the nervous tempera-
ment, the extent of surface covered by the floor of the chamber is much
less than that occupied by the occlusal portion. In the former class,
the entrances to the various canals are much farther apart than in the
latter. The occluding wall is usually much thicker than the lateral
walls, and is penetrated by the horns of the pulp-chamber, one of which
extends into each cusp. As in the bicuspids, the extent to which the
horns penetrate the cusps is controlled by the prominence of the latter.

The floor of the pulp-chamber is irregu-
c larly rounded, being high in the corner
and gradually falling away in the direc-
tion of the canals. The entrances to the
B 1 root-canals, three in number, are placed
in the form of an irregular triangle, called
t|L ' M the molar triangle. The mesial side of

A "\ the triangle is usually the longest, the distal

|^te^ next in length, and the buccal the shortest.

In young teeth the entrances to the canals
are usually in the form of funnel-shaped
openings, are comparatively easy of access,
but after maturity may disappear and be but little larger than the canals
themselves. To properly study the position occupied by the entrance
to the canals on the floor of the pulp-chamber, a transverse section of
the tooth should be made at a point somewhat above the cervical line,
at the same time preserving both the crown and the roots of the tooth
for comparison. The entrance to the lingual canal, which is usually
the largest and most readily accessible, may be located by a line drawn
through the center of the occlusal surface of the crown (Fig. 156) from
buccal to lingual, A, and by another line drawn from mesial to distal
almost parallel with the linguomarginal ridge, passing through the
summits of the mesiolingual and distolingual cusps, B; the point at which
these two lines intersect will mark the approximate location of the lingual
canal. The entrance to the mesiobuccal canal may be located by a line
drawn from the inner side of the mesiobuccal angle to a corresponding
position near the distobuccal angle, C. This should be intersected
by a line drawn from the summit of the mesiobuccal cusp to the summit
of the mesiolingual cusp, D, the point at which these two lines cross
marking the entrance to the mesiobuccal canal. The location of the

THE PULP-CAVITIES OF THE UPPER TEETH. 213

entrance to the distobuccal canal is found by the line, C, which is inter-
sected by another line, E, drawn from the summit of the distobuccal
cusp to a corresponding point on the distolingual cusp. The nearer
the tooth-crown approaches to the quadrilateral, the nearer will the
molar triangle approach the equilateral.

Upper First Molar. — In the dissection of this tooth, the pulp-
chamber and two of the root-canals only can be shown, but these will
be sufficient to pursue the study with intelligence. Figure 157 shows
a number of longitudinal sections, made in such a manner as to expose
the lingual canal, usually the largest, and the mesiobuccal canal. No. 1
illustrates the approximate size and form of the pulp-chamber at the

Fig. 157. — Pulp-cavity of Upper First Molar, from the Fifth to the Ninth
Year. Lingual and Mesiobuccal Canals.

fifth year. At this period the chamber occupies a large proportion of
the center of the tooth-crown. Two of the four horns are seen, one of
which penetrates the mesiobuccal cusp, and one the mesiolingual cusp.
In many instances the horns of the molar teeth are quite slender, penetrat-
ing the dentin to a greater depth than that shown in the illustration, in
the form of minute hair-like projections, which in some instances reach
almost to the enamel walls.

No. 2 illustrates the condition of the pulp-cavity at the sixth year,
or at the time of eruption. The outline of the pulp- chamber is completed,
and the floor has begun to make its appearance by a central deposit
of dentin. It will be observed that the lateral walls of the chamber are
somewhat less in thickness than the occluding wall, a condition which
will become more pronounced as the tooth develops. With the beginning
of the formative process in the floor of the chamber we find the trifurca-
tion of the roots established, and the beginning of the canals outlined.
The canals at this period are quite similar to those of the bicuspid, being
provided with a funnel-shaped extremity, which extends from the free

214 • ANATOMY.

calcifying margins of the roots to the floor of the chamber. No. 3 shows
the change which has taken place at the seventh year. While the pulp-
chamber is somewhat reduced in size, but little change is noticeable
in its outline. By this time the floor of the chamber has become an
important factor in the tooth development. By the constant lateral
extension of this central deposit of dentin the floor of the chamber is
gradually spread out, this alteration being at the expense of the entrances
to the root-canals, which become reduced in diameter as the floor is
extended. The horns of the chamber are slightly less prominent, but
this part of the cavity has the appearance of having receded bodily root-
ward. The roots have advanced somewhat beyond the point of trifurca-
tion, and a definite outline has been given to the canals. At this period
the diameter of the root-wall is about equal to the diameter of the pulp-
canal. Along with the gradual decrease in the diameter of the roots,
there is observed a corresponding decrease in the width of the funnel-
shaped extremities of the canals.

At the eighth year, No. 4, a gradual reduction in the capacity of
both the chamber and canal is noted. Accompanying the above con-
dition there is found a corresponding increase in the thickness of the
surrounding walls. The horns of the pulp-chamber are much reduced
in size, and the form of the chamber more closely resembles that of the
general contour of the tooth-crown. The increase in the length of the
roots is proportionately greater than that of previous years, in conse-
quence of which the length of the canals is increased to a greater degree.
In No. 5 the maximum size of the chamber and canals is apparent, which
condition takes place about the ninth year, or at a time when calcification
of the tooth is completed externally. In some instances, owing to the
additional length of the lingual root, the apical foramen may not be
established before the tenth year. At this latter period it is safe to assume
that all three canals have completed their longitudinal extent, and the
foramina, although proportionately large, have been established, so
that a more definite description of each canal may be given. The lingual
canal (Fig. 158, A) is usually the largest, and branches off from the floor
of the chamber, near the mesiodistal center of the extreme lingual margin,
the entrance in the average tooth being well defined by a circular, funnel-
shaped opening. The direction of this canal is usually upward and
slightly inward, until the apical extremity is approached, at which point
it is inclined to the buccal. The circular form presented at the beginning
of the canal is generally continued throughout its entire length, in this
respect differing from the two buccal canals. The average length of

THE PULP-CAVITIES OF THE UPPER TEETH. 215

the lingual canal is about 1/2 of an inch. The mesiobuccal canal (Fig.
158, B) branches off from the floor of the chamber, at its extreme mesio-
buccal angle, and the entrance, instead of being funnel-shaped and easy
of access, is flattened from mesial to distal, and frequently difficult to
enter. This flattened form continues throughout its course, which for
the distance of 18 of an inch is in a buccal and mesial direction; beyond
this point it is usually inclined to the buccal, until the upper thrid of the
root is reached, where it turns rather abruptly to the distal. This canal
is generally a trifle shorter than the lingual, averging about 3/8 to 7/16 of

D E F

Fig 158. — Pulp-cavities of Upper First Molar, Slightly Enlarged.

an inch. The distobuccal canal (Fig. 158, C) branches off from the floor
of the chamber at the extreme distobuccal angle. In those teeth which
most nearly approach the quadrilateral form, the entrance to this canal
will be farther from the center of the tooth, the molar triangle in this
instance being almost an equilateral. It sometimes happens that the
entrance to this canal is directly in the floor of the pulp-chamber, near to,
but not against, its buccodistal angle. The entrance is usually abrupt,
seldom being funnel-shaped, making it by far the most difficult of access.
It is inclined to be circular in form, and more or less tortuous in its course.
Immediately above the point of beginning it is inclined toward the buccal
and distal; near its center it may incline slightly to the mesial; and

2l6 ANATOMY.

finally, at its upper third, turns somewhat abruptly in a distobuccal
direction. This canal is usually the shortest of the three, its average
length being about 3/8 of an inch.

Figure 158 also illustrates a number of transverse sections of this
tooth, D being made at the cervical line, looking toward the crown, E
looking toward the roots, while F represents a transverse section at a
point immediately above the floor of the pulp-chamber.

Upper Second Molar. — In many respects the pulp-chamber of
this tooth is similar to that of the first molar, but there are a few variations
which must be briefly described. First, the outline of the tooth-crown

9th year nth year 1.3th year 15th year 1 8th year

Fig. 159. — Pulp-cavities in the Upper Second M'>!ar,'from the Ninth to the

Eighteenth Year.

being much more flattened from mesial to distal, a corresponding varia-
tion is noted in the form of the pulp-chamber, increasing the length of
the mesial side of the molar triangle, and decreasing the length of the
buccal and distal sides. The chamber is more or less flattened from
mesial to distal, making it somewhat oblong from buccal to lingual.
Second, on account of a reduction in the prominence of the cusps, the
horns of the cavity are usually somewhat less pronounced than those of
the first molar. Third, the floor of the cavity is less convex, and slopes
more gradually toward the entrances of the various canals. In a general
way, the rules given for ascertaining the approximate location of the
entrances to the canals in the first molar apply to this tooth. The com-
parative size and form of the pulp-chamber and canals during the develop-
ment of the tooth are shown in figure 159, extending from the ninth to
the sixteenth or eighteenth year, at which latter period the crown and
roots of the tooth are fully calcified externally.

Upper Third Molar. — In this tooth the conditions are so variable
that a description of the pulp-cavity taken from a single tooth would be
insufficient. In the majority of instances the outline of the tooth-crown

THE PULP-CAVITIES OF THE UPPER TEETH. 217

approaches the triangular form, and in consequence the pulp-chamber
is triangular rather than quadrilateral or oblong. The mesial border
of the chamber is the longest, the distal next in length, and the buccal
the shortest of the three. The horns are generally less in number and
much less pronounced than those of either the first or second molars.
The floor of the chamber may be broken by irregularities similar to
those previously described, or it may be entirely absent, this latter con-
dition occurring when the tooth has but a single root accompanied by a
single canal. The various stages of development having been given in
connection with the general description of the tooth, no attempt will

ABC

Fig. 160. — Longitudinal Sections, Upper Third Molar, Slightly Enlarged.

be made to describe this by longitudinal sections, the complications in
root-form making such a proceeding impracticable. Instead of so doing,
the space will be devoted to a brief description of the variety of pulp-
canals found in this tooth. Probably the most frequent condition is
that which resembles the first and second molars — i.e., three canals
branching off from the chamber in as many different roots, two to the
buccal and one to the lingual. When the three canals exist, the entrances
to them will be well beyond the cervical line, where they will be found
clustered much closer together than those of the first and second molars,
this difference in their location being so marked that the diagram pre-
viously given cannot be depended upon in an attempt to locate them.
The usual course of these canals is first slightly mesial, then distal, and
finally in a distolingual direction. On account of the pulp-chamber
extending well beyond the cervical line, the canals are much shorter than
those of the first or second molars, their average length being less than
1/2 of an inch. Another form frequently met with is that of the flattened
single canal, occurring when the tooth has but a single root, which shows

2l8 ANATOMY.

no signs of trifurcating (Fig. 160, A). In this instance the pulp-chamber
gradually passes into the canal, and the chamber is without a floor. Such
a canal is shaped like the chamber at its point of beginning; but as it
passes toward the apex it becomes flattened in the direction of the smallest
diameter of the root. But little difficulty is experienced in entering
such a canal, and usually it is readily followed to its apex. Another
condition frequently met with in the single-rooted third molar is that
of one or more canals branching off from the floor of the chamber, their
course through the root-substance being without regard to the external
contour of the root (Fig. 160, B). These canals, which may exist to
the number of five or six, are usually very minute, and in some instances
may pass from the floor of the chamber to the apex of the root almost
in a direct line, and end in distinct foramina, or they may take a tortuous
course, and when near the apex unite, ending in a single foramen.' When
the tooth is provided with four, five, or even six small roots, as sometimes
occurs, each root will be traversed by a minute canal, the entrances to
these being variously placed about the floor and lateral margins of the
pulp-chamber (Fig. 160, C). In all operations upon this tooth it must
be recalled that it is the last to be calcified, and consequently the canals
and foramina are proportionately larger than in the other teeth; at the
same time, it possesses one advantage over the others — i.e., (with the
single exception of the cuspid), being fully calcified at or about the time
of its eruption.

PULP-CAVITIES OF THE LOWER TEETH. 2I(

PULP-CAVITIES OF THE LOWER TEETH.

A B C

Fig. 161. — Pulp-cavities of the Lower Incisor.

The outline of the pulp-cavities of the lower teeth, like those of the
upper, corresponds to the general tooth contour. The comparative size
of the cavity at various stages of tooth development will not be repeated
in this description, the conditions being similar to those in the upper
teeth (see also Development of the Teeth).

Lower Incisors. — The pulp-cavities of the lower central and
lateral incisors are so nearly alike that a single description will answer
for both. Figure 160, A, represents a labiolingual section of a lower
incisor, showing the most frequent form of the pulp-cavity. The tooth
from which the section was prepared was one about middle life, the
cavity in younger teeth being proportionately larger, while a gradual
decrease in diameter would be noted with advancing age. There is no
mark of distinction between the pulp-chamber and canal, so that an
imaginary separation would have to be made at the cervical line, or
slightly below that point. Taken in its entirety, the cavity presents the
form of a double cone, the common base of which is slightly to rootward
of the cervical line. The chamber penetrates the crown fully half-way
to the cutting-edge, at which point it ends in a thin, fan-like margin
(best observed in mesiodistal section), while the canal gradually decreases
in size until the apical foramen is reached. Although this is the most
common form of the pulp-cavity in the lower incisors, it is by no means
the constant condition. The tooth is not infrequently provided with a
medium-sized pulp-chamber, which extends somewhat below the cervical

220 ANATOMY.

line, beyond which point it branches into fine canals, which are continued
separately until the apical third of the roots is approached, when they
again unite, and finally end in a single foramen. Figure 161, C, repre-
sents a mesiodistal section of a young lower incisor, in which the three
small horns of the pulp-chamber are apparent. At this period the fan-
shaped extremity of the pulp-chamber occupies about one-half of the
mesiodistal diameter of the crown, and the horn-like projections extend
well toward the enamel cap. Figure 161, B, shows the average size and
form of the pulp-cavity at maturity, by a mesiodistal section through the
long axis of the tooth. In this it will be observed that the horns of the
pulp-cavity have disappeared, and that the capacity of the cavity in
general is much reduced.

Fig. 162. — Pulp-cavities of the Lower Cuspids.

Lower Cuspids. — The pulp-chamber and canal in this tooth, while
usally conforming to the general contuor of the tooth, are frequently found
to vary greatly, both in outline and in size. The most common form,
however, is that shown in Fig. 162, A, a labiolingual section of an adult
tooth. The chamber and canal have no line of demarcation, and unite
at a common base considerably below the cervical line, the former pene-
trating the crown of the tooth to a point about midway between the cer-
vical line and the summit of the cusp, at which point it ends in a sharp,
hair-like projection. Accompanying this common form there is much

PULP-CAVITIES OF THE LOWER TEETH.

221

variation in size, even in teeth of the same age. Fig. 162, B, shows
another labiolingual section of an adult lower cuspid, in which the pulp-
cavity fails to accurately follow the outline of the tooth, and its capacity
is much less than that shown at A. The root of this tooth is in most
instances circular, in which case the canal will be similarly formed; but
occasionally the root will be much flattened from mesial to distal, and
as a result of this the canal will also be much flattened. The canal of
this tooth is seldom divided. In Fig. 162, C, a mesiodistal section of a
lower cuspid is shown, and it will be observed that the fan-shaped ex-
tremity of the chamber common to the incisors is absent, the cavity end-
ing rather abruptly, or by a fine line near the center of the crown. The
same illustration also shows a number of transverse sections, which will
give an idea of the form of the cavity at various parts of the tooth.

Lower Bicuspids. — The pulp-cavities of these teeth may be best
described collectively, thus affording an opportunity for comparison. Unlike

Fig. 163. — Sections of Lower Bicuspids.

the upper bicuspids, it is seldom that the canals are definitely separated
from the chambers. That part of the cavity within the crown, however,
is usually quite wide from buccal to lingual, and unites with the canal by
a long, funnel-shaped constriction. The center of the pulp-chamber
may be considered as being about on a level with the cervical line. In
the first bicuspid the pulp-cavity is provided with a single horn, which
extends with more or less prominence in the direction of the buccal cusp.
That part of the chamber facing the lingual cusp is usually rounded off.

222 ANATOMY.

In the second bicuspid the occlusal wall of the pulp-chamber generally
presents a different form; two well-defined horns are usually present, of
which the buccal is the longest; or the chamber may be prominently
rounded at these points. The pulp-chamber of the second bicuspid is
generally larger than that of the first. The canals of these teeth are
usually circular throughout, and are readily penetrated until the apical
third is reached, beyond which point they are extremely small. In some
instances the canal divides near the center of the root, and is continued
as two canals, ending in distinct foramina, or, after separating, they may
again unite, and end in a single foramen. In Fig. 163 the average size
and form of the canal in these teeth is shown by a number of mesiodistal
and transverse sections.

FlG. 164. — Sections of Lower Molars, Enlarged about One-third.

Lower Molars. — The form of the pulp-chambers of the lower molars
corresponds to the general outline of the crown, and the form of the root-
canals is similar to the general contour of the roots. The pulp-chambers
approach the quadrilateral form; the buccal and lingual sides are some-
what the longest, the mesial next in length, and the distal, usually slightly
rounded, is the shortest. The occluding wall is convex rootward, sloping
in the direction of the various cusps, each of which is penetrated by a horn.
Like the horns of these pulp-chambers in general, the extent to which these

PULP-CAVITIES OF THE LOWER TEETH. 223

penetrate the cusps is influenced by the age, type, and functional activity of
the organ. The floor of the cavity is convex in the direction of the occlusal
surface, but this convexity is principally from mesial to distal. From the
summit of this convexity the floor slopes to the entrances of the canals, the
opening into which is inclined to be funnel-shaped rather than abrupt.
The lateral walls of the chamber are much inclined to follow the general
contour of the crown. The horns of the pulp-chamber are usually more pro-
nounced in the first than in the second molar, and still less clearly defined
in the third than in the second. The roots of the first molar being some-
what further apart than those of the second, the floor of the chamber in
the former is slightly more extensive than in the latter. To study the
pulp-cavities of these teeth a longitudinal section should be made through
the center of the tooth from mesial to distal. Fig. 164, A, shows such a
dissection through the first molar, and illustrates the average size and
form of the chamber and canals at adult age. The canals join the
chamber by a funnel-shaped opening, and but little difficulty will be found
in effecting an entrance, but to follow them to their apices will be more
perplexing. The roots of this tooth being much flattened from mesial to
distal, the canals are also flattened in this direction, but broad from buccal
to lingual. The entrances of these canals may be found at the extreme
mesial and distal margins of the pulp-chamber, and usually extend from
the buccal to the lingual walls of the cavity. It is not uncommon for the
mesial canal to divide soon after leaving the chamber, and continue as
two canals, ending in separate foramina (Fig. 164, B). This condition
is seldom present in the distal canal, which is usually straight from its
mouth to the apical foramen. The capacity of the pulp-chamber is
usually a trifle less than that of the first molar, and the entrances to the
canals are somewhat nearer together. In other respects the cavity is simi-
lar to that of the first molar. Fig. 164 also shows a number of transverse
sections through a lower molar, and gives an idea of the size of the canals
at various parts of the roots. In some instances the roots of the second
molar coalesce, in which case a single root-canal may be present. In
the third molar the most common form of the pulp-cavity is one similar
to that of the first, but both the chamber and canals are smaller. Unlike
the pulp-cavity of the corresponding upper tooth, this tooth is not sub-
ject to so much variation, although it is sometimes found with a single
root traversed by a single canal, which may be accompanied by a rather
large pulp-chamber.

CHAPTER XL

The Deciduous Teeth, Their Arrangement, Occlusion, Etc. ; Their
Calcification, Eruption, Decalcification, Shedding Process,
and Average Measurements ; Their Surfaces, Grooves, Fossae,
Ridges, Sulci, and Pulp-Cavities.

THE DECIDUOUS TEETH.

Central Incisor Lateral Incisor

Cuspid

First Molar

Second Molar

Upper

Lower

Central Incisor Lateral Incisor Cuspid First Molar Second Molar

Fig. 165. — The Deciduous Teeth, Upper and Lower, from the Left Side of the Mouth.

As implied by the word deciduous, these teeth are temporary in their
nature, and, after subserving the purposes of early childhood, are thrown
off by an operation of the economy to give place to the permanent organs.
The shedding process takes place in the incisors between the seventh
and eighth years, in the molars from the tenth to the eleventh years, and
in the cuspids about the twelfth year. This shedding process, however,

224

THE DECIDUOUS TEETH. 225

does not indicate the period at which the degeneracy of the tooth begins,
for, in a year or two after the roots are completely formed and the apical
foramen established, decalcification begins at the apical ends and con-
t inues in the direction of the crown until absorption of the entire root has
ta ken place and the crown is lost from lack of support. Decalcification
in the incisors begins between the fourth and fifth years, in the molars
from the seventh to the eighth years, and in the cuspids about the ninth
year.

The deciduous teeth are twenty in number, ten in each jaw, and may
be classified as follows: Four incisors, two cuspids, and four molars.
The incisors, central and lateral, occupy the central portion of the arch,
are placed two upon each side of the median line, and are succeeded by
the four permanent incisors, which finally occupy the same position.
The cuspids are located immediately to the distal of the lateral incisors,
and are displaced by the permanent cuspids. The first and second molars
come next in the arch, but, unlike the anterior teeth, are followed by per-
manent successors of another class, the first and second bicuspids, the
permanent molars erupting posteriorly to these as the jaw increases in
length.

In general the deciduous teeth resemble their permanent successors,
yet there are a number of minor differences which will require a compara-
tive description. Both the crowns and the roots are much smaller in
every direction than those of the permanent teeth, but the diameter of
the crowns is proportionately greater than that of the roots, while the
roots are proportionately longer. The fact that the roots are smaller in
proportion than the crowns is productive of a neck much more constricted.
The roots of the deciduous teeth are the same in number as those of the
corresponding permanent teeth, the incisors and cuspids being provided
with one, the upper molars with three, and the lower molars with two.

226

ANATOMY.

THE OCCLUSION OF THE DECIDUOUS TEETH.

Cuspid

Incisors

First Molar
Second Molar

Permanent First
Molar

Fig. 166. — The Upper Dental Arch about the Seventh Year.

The arrangement of the deciduous teeth in the jaws is similar to that
of the permanent organs, the upper teeth describing the segment of a
larger circle than the lower, in consequence of which the upper teeth
close over or outside of the lower. The character of the occlusion in the
deciduous teeth is not subject to so much variation as that found in con-
nection with the permanent set, this being accounted for by the more
constant form in the crowns of the former. The relations existing between
the upper and lower deciduous teeth when in contact is such that each
tooth, with the exception of the lower central incisor and the upper second
molar, occludes with two teeth of the opposite jaw, the upper central
incisor being opposed by the entire cutting-edge of the lower central and
the mesial third of the lower lateral; the upper lateral coming in contact
with the remaining two-thirds of the lower lateral and a portion of the
mesial half of the lower cuspid, this arrangement continuing throughout
the series. The foregoing description of the occlusion of the deciduous
teeth is applicable to but a small part of their transitory existence. By
the time they are fully erupted and have assumed their respective posi-
tions in the arch, the increase in the size of the bone is sufficient to create
a slight space between the teeth, which condition is soon followed by a
greater separation through the protrusion of the anterior teeth, caused by
the growth and approach of the permanent teeth from behind.

The calcification of the deciduous teeth is similar to that of the per-
manent, the process of the incisors and cuspids beginning along the cut-

THE DECIDUOUS TEETH IN DETAIL.

227

ting-edges in three distinct lobes, while in the molars a center of calcifi-
cation is provided for each cusp (see Development of the Teeth).

THE DECIDUOUS TEETH IN DETAIL.

UPPER CENTRAL INCISOR.

Eighteenth Month
after Birth

Sixth Month after
Birth

Fortieth Week

Twentieth Week

Fourth Year

Fifth Year

Seventh Year.

O
c

Fig. 167.

Calcification Begins, about the Fourth Fetal Month.

Calcification Completed, Seventeenth to Eighteenth Month after Birth.
Erupts, Sixth to Eighth Month after Birth.
Decalcification Begins, about the Fourth Year.

Shedding Process Takes Place, about the Seventh Year.
Average Length of Crown, .23.

Average Length of Root, .39.

Average Length over All, .62.

This tooth, as well as all of the deciduous teeth, presents for examina-
tion numerous surfaces, margins, and angles, these being the same in
name and location as those of the permanent teeth.

The Labial Surface of the Crown (Fig. 168). — This surface is smooth
and generally convex, but with an inclination to flatness near the incisive
margin. The mesial margin is slightly convex in the direction of the
length of the tooth, and rounded from labial to lingual. The distal mar-
gin is decidedly convex from the cutting-edge to the cervical line, in many
instances forming almost a complete semicircle, which is usually at the
expense of the distal angle of the crown. The cervical margin is deeply
concave in the direction of the root, and the incisive margin is straight over
its central portion and rounded or angular at its extremities. The labial
grooves are seldom so well defined as those upon the permanent incisors.

The Lingual Surface of the Crown. — In some instances this
surface is smooth and concave near the cutting-edge and convex over the
cervical portion, with the marginal ridges well defined. In other cases it is
concave from the cutting-edge to the cervical ridge, being provided with a
longitudinal ridge in the center, a slight depression upon either side, and

228

ANATOMY.

marginal ridges poorly denned. In the former instance the lingual
fossa is present; in the latter it is absent. The mesial and distal sur-
faces of the crown are both smooth and convex, the
former being inclined to flatness over its cervical
third — a condition which is seldom present in the
latter. The mesialangle is alone well defined, the
cutting-edge passing into the distal surface with a
long, gradual sweep, thus in a measure destroying
the distal angle. The neck of the tooth is marked
by a decided constriction, which is principally pro-
duced at the expense of the crown alone. The
root of the tooth, when compared with the root of
the permanent central incisor, is much longer in
proportion to the length of the crown. In some
instances it is flattened from mesial to distal,
these two sides converging as they pass to the lingual; in others it is flat-
tened from labial to lingual. Generally speaking, it is a single root,
but is occasionally provided with a slight mesial curve near its apical
third, and it is sometimes curved slightly from labial to lingual.

Fig. 168.

UPPER LATERAL INCISOR.

Sixteenth Month
alter Birth

Sixth Month after
Birth

Fortieth Week
Twentieth Week

Fifth Year

Seventh Year

Eighth Year

31
n
P

Fig. 169.

Calcification Begins, about the Fourth Fetal Month.

Calcification Completed, Fourteenth to Sixteenth Month after Birth.
Erupts, Seventh to Ninth Month after Birth.
Decalcification Begins, about the Fifth Year.

Shedding Process Takes Place about the Eighth Year.
Average Length of Crown, .25.

Average Length of Root, .45.

Average Length over All, .70.

The various surfaces of this tooth so closely resemble those of the cen-
tral incisor that a separate description will be unnecessary; in a general

THE DECIDUOUS TEETH IN DETAIL.

229

way, however, there are a few minor points of
difference. The tooth is smaller in every direc-
tion excepting in its length, which is generally
equal to and frequently greater than that of the
central incisor. The diameter of the root is but
little less than that of the central, while the
mesiodistal measurement of the crown is about
one-third less, in consequence of which the neck
of the tooth is not so well defined. The angles of
the crown are more rounded than those of the
central incisors.

Fig. 170 — Upper Lat-
eral Incisor, Labial Sur-
face.

UPPER CUSPID.

« 1
o

Second Year

Sixth Month
after Birth

Bir.h

Thirtieth Week
Embryo

Ninth Year

Tenth Year

Twelfth Year

Fig. 171.

Calcification Begins, about the Fifth Fetal Month.

Calcification Completed, about Two Years after Birth.

Erupts, Seventeenth to Eighteenth Month after Birth.
Decalcification Begins, about the Ninth Year.

Shedding Process Takes Place, about the Twelfth Year.
Average Length of Crown, .25.

Average Length of Root, .53.

Average Length over All, .78.

Like the permanent cuspid, the general contour of this tooth is that of
a double cone, the lines of which are somewhat broken. The greatest
mesiodistal extent of the crown is from angle to angle, and this measure-
ment about corresponds with the width of the crown of the central incisor.

The Labial Surface of the Crown (Fig. 172). — This surface is
strongly convex from mesial to distal, and slightly so from the cutting-edge
to the cervical line. It is bounded by five margins: mesial, distal, cervical,

230

ANATOMY.

mesio-incisive, and disto-incisive. The mesial and distal margins are
rounded and smooth, the cervical well outlined by the cervical line and
base of the cervical ridge, while the two incisive margins are formed by
the mesial and distal cutting-edges. The labial grooves are thrown well
toward the lateral margins, and are usually more distinct than those
upon the incisors. The labial ridge is prominent.

The Lingual Surface of the Crown. — This
surface is generally divided into two portions by
the lingual ridge, which extends from the sum-
mit of the cusp to the base of the cervical ridge.
On either side of this ridge are the lingual
grooves, but which appear more in the form of
small fossae. The marginal ridges are fairly well
defined.

The Mesial and Distal Surfaces of the
Crown. — The extent of these two surfaces is fre-
quently much interfered with by the slope of the
mesial and distal cutting-edges, which may be so
long that the angles of the crown are forced
well toward the cervical line, in some instances almost obliterating
these two surfaces. When the cutting-edges are shorter, these sur-
faces present a marked general convexity. While the summit of the cusp
will always be found to be in a direct line with the long axis of the tooth,
there is in nearly every instance a difference in the length of the cutting-
edges, and, unlike the cutting-edges of the permanent cuspid, the mesial
is usually the longer. The neck of the tooth is much constricted and
the root straight and conic.

Upper Cuspid,

Labial Surface.

THE DECIDUOUS TEETH IN DETAIL.

231

THE UPPER MOLARS.

Upper First Molar.

Twenty-two
Months Old

Eighteen
Months
Old

One Year
Old

Six Months

after Birth

, Forty Weeks

Six Years
Seven Years

Nine Years
Ten Years

a
o

Si
o
P

Fig. 173.

Calcification Begins, about the Fifth Fetal Month.

Calcification Completed, Eighteenth to Twentieth Month after Birth.
Erupts, Fourteenth to Fifteenth Month after Birth.
Decalcification Begins, Sixth to Seventh Year.

Shedding Process Takes Place, about the Tenth Year.
Average Length of Crown, .20.

Average Length of Root, .39.

Average Length over All, .59.

The contour and lobate construction of the crown ofjthis tooth is
peculiar to itself, being dissimilar to any other class of teeth in the mouth.
Calcification takes place from three centers, two for the buccal and one
for the lingual half of the crown. The general form of the crown may
best be studied by an examination of the occlusal surface.

The Occlusal Surface of the Crown. — The outlines represented are
those of an irregular quadrilateral, of which the buccal and mesial sides
are the longest. The angles of the quadrilateral are somewhat variable,
the mesiobuccal being acute, the mesiolingual obtuse, while the two
distal angles are rounded right angles. The surface is surmounted by
three cusps, a mesiobuccal, a distobuccal, and a lingual. These various
cusps are separated from one another by three developmental grooves — the
mesial, the distal, and the buccal. The marginal ridges are sharp and
well defined, this being particularly true of the buccal and lingual, which
resemble cutting-edges. The mesio-marginal ridge begins at the
mesiobuccal angle, and, after making a long distal curve, ends in the
mesial incline of the lingual cusp. The center of the surface is deeply
and irregularly concave, producing the central fossa, and descending
from the various ridges and cusps surrounding it are numerous supple-
mental grooves and ridges. The various developmental grooves are not
inclined to cross the marginal ridges, although in some instances one or
two may be found to do so.

232

ANATOMY.

The Buccal Surface of the Crown (Fig. 173). — This surface is
generally smooth and convex, with an excessively developed cervical ridge,
which is particularly prominent at its mesial extremity. The buccal
groove is in the form of a slight depression, and the buccal ridges, common
to all molars, are scarcely to be observed. The mesial, occlusal, and cer-
vical margins are distinctly outlined, while the distal margin is obliterated
by the gradual passing of this surface into the distal surface.

The Lingual Surface of the Crown. — This surface is circular in
outline, decidedly convex and smooth, and is seldom broken by grooves

and ridges. It is most prominent
near the center, from which point
it slopes in every direction. The
cervical ridge is not so pronounced
as that of the buccal surface, but
there is a sudden rounding of the
surface in a cervical direction to
meet the lingual root.

The Mesial Surface of the
Crown. — This surface is probably
more extensive than any of the
others; it is inclined to flatness,
with a slight conic convexity over
its occlusal third, and a slight con-
cavity near the cervix. The buc-
colingual measurement of the sur-
face is nearly twice as great as that from the occlusal margin to the
cervical line. It is much more prominent near the occlusal margin, so
that a V-shaped space usually exists between it and the distal surface of
the cuspid.

The Distal Surface of the Crown. — The extent of this surface
is much less than that of the mesial; it presents a general convexity, and
is seldom broken by grooves or ridges, although occasionally the distal
groove crosses its occlusal margin. Like the deciduous teeth previously
described, the neck of the tooth is marked by a decided and abrupt con-
striction, this form appearing to arise from the heavy enamel folds which
are present near the cervical line, rather than from any marked con-
striction in the base of the roots themselves.

The roots of the tooth are three in number — a mesiobuccal, a disto-
buccal, and a lingual; of these, the latter is usually the largest and longest.
The two buccal roots are much flattened from mesial to distal, while the

Fig. 174. — Occlusal Surfaces of the

I )c( iiluous Millars.

THE DECIDUOUS TEETH IN DETAIL.

2 33

lingual is compressed in the opposite direction. The apical ends of the
roots are much separated from one another, the triangle which these
points form being almost twice the size of the triangle formed by the base
of the roots. The apical ends are usually provided with a central curve.

Upper Second Molar.

c3
U

Twenty-two
Months Old

One Year Old

Six Months
after Birth

Birth

Eight Years

Ten Years

Eleven Years

Fig. 175.

Calcification Begins, between the Fifth and Sixth Fetal Months.

Calcification Completed, Twentieth to Twenty-second Month after Birth.
F.rupted; Eighteenth to Twenty-fourth Month after Birth.
Decalcification Begins, Seventh to Eighth Year.

Shedding Process Takes Place, Eleventh to Twelfth Year.
Average Length of Crown, .22.

Average Length of Root, .46.

Average Length over All, .68.

The most remarkable feature about the crown of this tooth is its close
resemblance to the crown of the upper permanent first molar. The
various surfaces are almost identical, the de-
velopmental process, and consequently the cusp-
formation, is the same, the marginal and other
ridges common to the occlusal surface corre-
spond, and both the central and distal fossae are
present, together with the various developmental
grooves. A description of the crown will, there-
fore, be unnecessary; suffice it to say that it is
much smaller in every direction and is some-
what more constricted at the neck. The roots
are the same in name and number as those of
the first permanent molar, but they are more
widely separated at their apical extremities. In general form they are
smaller than those of the upper first deciduous molar.

Fig. 176.
ond M o
Surface.

—Upper Sec-
a r, Buccal

234

ANATOMY.

THE LOWER DECIDUOUS TEETH.

A description in detail of the lower incisors and cuspids would practi-
cally be a repetition of that given of the corresponding upper teeth, and
for that reason will be passed with a limited reference to each. The
lower molars being in many respects unlike the upper, they will require
a separate description.

Lower Central Incisor (Fig. 177).

Calcification 1 Begins, about the Fourth Fetal Month.

Calcification Completed, Sixteenth to Eighteenth Month after Birth.
Erupted, Sixth to Eighth Month after Birth.
Decalcification Begins, about the Fourth Year.

Shedding Process Takes Place, about the Seventh Yeas.
Average Length of Crown, .19

Average Length of Root, .35.

Average Length over All, .54.

Fig. 177 —
Lower Central
Incisor, LaMal
Surface.

This is the smallest of the lower teeth, in this
respect being at variance to the upper central, which
is larger than the lateral. The mesiodistal diameter
of the crown is but little less than that from the
cutting-edge to the cervical line. The mesial and
distal angles are similar, both being pointed and
square. The cervical ridge is quite pronounced and
the neck much constricted.

The root is usually straight and tapers gradually
from base to apex. It is broader on the labial than
on the lingual side, and the mesial and distal sides
are but little flattened.

Lower Lateral Incisor (Fig. 178).

Calcification Begins, about the Fourth Fetal Month.

Calcification Completed, Twelfth to Fourteenth Month after Birth.
Erupted, Seventh to Ninth Month after Birth.
Decalcification Begins, about the Fifth Year.

Shedding Process Takes Place about the Eighth Year.
Average Length of Crown, .19.

Average Length of Root, .39.

Average Length over All, .58.

This tooth is larger than the central incisor, and closely resembles the
upper lateral both in size and form. The crown is more rounded in its

THE DECIDUOUS TEETH IN DETAIL.

2 35

nature than that of the central, forming a greater general convexity to
the labial surface, and less concavity to the lingual. The mesial angle of
the crown is fairly well defined, while the distal is
usually much rounded by a long, circular sweep of
the cutting-edge to meet the distal surface.

The mesial surface of the crown is flattened and
somewhat prominent at the angle, while the distal
surface is strongly convex. The labial grooves are
but slightly visible, while the corresponding lingual
grooves are quite pronounced. The neck of the tooth
is even more marked than that of the lower central
incisor. The root is long and tapering, slightly flat-
tened from mesial to distal, with a decided longitudinal
groove on both the mesial and distal sides. The
labial and lingual sides are rounded, and there is an
inclination to crookedness, which is usually from mesial to distal.

Fig. 178. —
Lower Lateral
Incisor, Labial
Surface.

Lower Cuspid (Fig. 179).

Calcification Begins, about the Fifth Fetal Month.

Calcification Completed, about Two Years after Birth.

Erupted, Seventeenth to Eighteenth Month after Birth.
Decalcification Begins, about the Ninth Year.

Shedding Process Takes Place, about the Twelfth Year.
Average Length of Crown, .23.

Average Length of Root, .45.

Average Length over All, .68.

Fig. 179. —
Lower Cuspid,
Labial Surface.

The principal variations between this tooth and
the upper cuspid are observed in the diminished
mesiodistal measurement of the crown, together with
it being somewhat less angular in outline. The
ridges and grooves common to the various surfaces
are not so marked as those of the upper cuspid, re-
sulting in a smoothly formed crown throughout. The
root is larger in proportion to the size of the crown
than that of its upper opponent, thus producing a
neck much less constricted. It is usually straight, or
provided with a slight distal inclination near its apical
extremity, and much flattened from mesial to distal,
these two sides converging to the lingual, forming a
rounded triangular outline.

236 ANATOMY.

Lower First Molar (Fig. 180).

Calcification Begins about the Fifth Fetal Month.

Calcification Completed, Eighteenth to Twentieth Month after Birth.
Erupted, Fourteenth to Fifteenth Month after Birth.
Decalcification Begins, Sixth to Seventh Year.

Shedding Process Begins, about the Tenth Year.
Average Length of Crown, .24.
Average Length of Root, .38.

Average Length over All, .62.

Upon making an examination of the occlusal surface of this tooth it
will be observed that the crown is made up of four irregularly formed lobes,
separated from one another by four well-defined grooves. Each lobe is

provided with a cusp, more or less prominently
developed. Between the various cusps are two
fossae — one occupying the distal two-thirds of the
surface (the distal fossa) and the other the re-
maining or mesial third (the mesial fossa).
The outline of this surface, which represents
the contour of the crown in general, is that of
an oblong square, with its angles more or less
Fig. 180.— Lower First rounded, and having a slight variation in its

Molar, Buccal Surface. _ . , ,

parallel lines. Each lobe denotes a separate
center of calcification, and the four grooves the lines of union between
the various parts.

The mesiobuccal lobe is somewhat irregular in contour and is fre-
quently the largest of the four. It assists in forming the mesiobuccal
angle of the crown and the greater part of the mesial fossa. Descending
from this cusp to the lingual is a pronounced triangular ridge, which is
made continuous by uniting with a similar ridge from the corresponding
lingual cusp. By this union a transverse ridge is established, separating
the mesial from the distal fossa. The central boundary of this lobe
is formed by the mesial groove, which arises from the distal fossa, passes
over the transverse ridge to the mesial fossa, from which it continues to
the lingual, and by the buccal groove, which branches off from the
mesial somewhat to the distal of the transverse ridge, passing over the
buccomarginal ridge to the buccal surface.

The Distobuccal Lobe. — This cusp is generally smaller than the
mesiobuccal, and is more pointed and more regular in outline. It assists
in forming the distobuccal angle of the crown, and by its central incline
forms about one-third of the distal fossa. Its boundaries are formed by
the buccal, mesial and distal grooves, the latter beginning in the distal fossa,
and passing over the distomarginal ridge to the distal surface.

THE DECIDUOUS TEETH IN DETAIL. 237

The Mesiolingual Lobe. — In the recently erupted tooth the summit
of this cusp is long and pointed, and frequently remains the most pro-
nounced of the four. It is triangular in outline, and, as above referred to,
furnished a triangular ridge, which, by uniting with a like ridge from the
mesiobuccal cusp, forms the transverse ridge. By its central incline it
assists in forming the mesial fossa. Its boundaries are formed by the
mesial groove and the lingual groove, the latter arising near the center of the
distal fossa, passing to, and sometimes crossing, the linguomarginal ridge.

The Distolingual Lobe. — This is usually the smallest of the four.
It is inclined to be rounded, rather than angular, and in some instances
is poorly developed. It assists in forming the distolingual angle of the
crown, as well as a portion of the distal fossa. Its central boundaries are
formed by the lingual and distal grooves.

The marginal ridges of the surface are abruptly but irregularly
formed, ascending and descending the various cusps in a manner similar
to those previously described.

The Buccal Surface of the Crown (Fig. 180). — This surface is
smooth and generally convex, with a mesiodistal measurement about
twice as great as that from the cervical line to the occlusal margin.
The surface is most prominent over its cervical third, forming a well-
rounded and bold cervical ridge, a feature strongly characteristic of this
tooth. The distal center of the surface is broken by the buccal groove,
which usually ends near the center in a shallow depression or pit.

The Lingual Surface of the Crown. — This surface is much less
extensive than the buccal. It is smooth and convex throughout,
and is broken near its distal center by the lingual groove, which gradually
disappears as it passes rootward. The cervical ridge is not so prominent
as that of the buccal surface.

The Mesial and Distal Surfaces of the Crown. — These are slightly
convex in every direction, the former passing, by a gradual sweep, into
the lingual surface, destroying the angularity of the crown at this
point, while the latter passes more abruptly, forming an acute angle.
These surfaces are both prominent near the occlusal margin, making the
point of contact with adjoining teeth near that surface. The bold cer-
vical ridge of the buccal surface is discontinued or greatly diminished
upon these surfaces, both of which are inclined to pass very gradually
into the base of the roots.

The roots of this tooth are the same in name, position, and number
as those of the lower permanent molars. They are much flattened from
mesial 'to distal, the center of their flattened sides being further com-

238 ANATOMY.

pressed by a deep longitudinal groove, which extends from the base to
the apex of each root. In passing from the base of the roots to their
apices they become more widely separated, until these extremities are
much wider apart, proportionately, than those of the permanent molars.

Lower Second Molar (Fig. 181).

Calcification Begins, between the Fifth and Sixth Fetal Months.

Calcification Completed, Twentieth to Twenty-second Month after Birth.
Frtjpted, Eighteenth to Twenty-fourth Month after Birth.
Decalcification Begins, Seventh to Eighth Yeak.

Shedding Process Takes Place, Eleventh to Twelfth Year.
Average Length of Crown, .21.
Average Length of Root, .44.

Average Length over All, .65.

The anatomy of this tooth being almost identical to that of the
lower first permanent molar, it will be unnecessary to enter into a descrip-
tion in detail. The lobes, and consequently the
grooves, are the same in position, name, and
number, and a similar developmental process is
recorded. The tooth is not characterized by a
prominent cervical ridge, such as is found upon
the lower first molar, the crown passing very
gradually into the root-base with a neck moder-
ately constricted.

THE PULP-CHAMBERS AND CANALS OF

" ~d THE DECIDUOUS TEETH.

Molar, Buccal Surface. A few general re marks in reference to these

cavities, in connection with the information to be derived from the
accompanying chart and its annexed description, will sufficiently instruct
the reader, without the necessity of treating each tooth individually.
The pulp-chambers and canals, like those of the permanent organs,
assume the form of the external contour of the tooth, the crown of the
tooth being provided with a central cavity, the pulp-chamber, partaking
of outlines closely resembling those of the crown, while the root is
traversed by the pulp-canal, likewise conforming to the shape of the
root. One very important distinction between the pulp-chambers and
canals of the deciduous teeth and those of the permanent organs is that
the former are proportionately larger. It must also be noted that the
apical foramina in these teeth are so transitory in their nature that there
remains but a very brief period during which the canals may be said
to be fully formed. It will be recalled that in a very short time after
the roots have become completely calcified, decalcification begins, and

THE DECIDUOUS TEETH IN DETAIL.

2 39

this process of degeneracy, beginning at the apical extremities of the
roots, very early destroys the foramina, which have in a measure served as
a protection to the surrounding parts during operations upon the canals.
With the canals proportionately larger than those which occupy the
roots of the permanent teeth, the foramina during their very limited
existence are also much larger, and much more readily penetrated.
With these ever-changing conditions in the pulp-canals of the deciduous
teeth, it is of importance that a definite knowledge of what takes place
should be acquired, and it is for this purpose that the accompanying
chart has been prepared (Fig. 182).

<5i

O 10

h-ts

3 §

C l—l

■a 3

c3 O

c
c o

d n,

~ Oh

a v

I u

uoiaBog!3^3

notjEogiojEoaQ

CHAPTER XII.

Development of the Teeth.— The Dental Germs, Enamel Organ,
and Dentin Organ; the Dental Follicle; Calcification and
Eruption.

DEVELOPMENT OF THE TEETH.

Oral Epithe
Hum

Infant Layer
of Cells

Stellate Retic

ulum

Ameloblasts

Dentin Papilla

Fig. 183. — Developing Tooth-germ. X 300.

In order that the student may obtain a general idea of the structural
changes which take place at a very early period, and which eventually result
in the formation of the teeth, the genesis of the subject will be briefly re-
ferred to. Preparation for the development of the teeth takes place as early
as the middle of the second fetal month, this preparatory alteration in the

240

DEVELOPMENT OF THE TEETH. 241

tissues beginning before the process of ossification in the bony structures
which finally surround and give support to the organs. At this early
period there will be found following the line of the future alveolar ridge
a slight heaping up of the surface epithelium, while immediately beneath
this proliferation of cells there appears a dipping in of the deep epithelial
layer in the direction of the future alveolar walls. This epithelial reflec-
tion is known as the epithelial band or tooth-band. This so-called tooth-
band is not, as might be supposed, a special inflection for each tooth-
germ, but is continuous from one end of the future jaw to the other.
It must be remembered that at this time the outline of the jaws has not
been established, and the tooth-band, although not generally considered
as essential to the developmental process, is principally instrumental
in directing the position of the dental organs. The position and form
of this epithelial band may best be studied in vertical transverse section.
When first making its appearance it is somewhat broad and shallow,
but as it passes more deeply into the parts it partakes of the outline of the
letter V with its open end directed toward the surface. In penetrat-
ing the subjacent tissue, the free extremity of the band is inclined to the
lingual, its external surface is slightly convex, and its internal surface
correspondingly concave. Histologically considered, the tooth-band is
composed of elements similar to those which serve to make up the
epithelial layer of the oral mucous membrane. After the tooth-band
has assumed certain proportions, there appears on its inner or concave
surface a thin membranous plate, which is likewise a continuous structure,
extending the full length of the epithelial band.

This lamina does not spring from the free margin of the tooth-band,
but is given off at a point about midway between this border and the base
of the band. The structure of this secondary band is so similar to that
of the primary one that it should be considered as an inflexion from it
rather than a new structure. We find them between the seventh and
eighth week, the maxillary regions giving place to two bow-shaped
bands (one for each jaw), each of which is preparing to throw out from
its secondary lamina ten little buds, which soon develop into the germs
for the twenty deciduous teeth. When these buds make their appear-
ance they are simple, rounded bodies, placed somewhat closely together,
but they do not long retain this simple form. The first change which
takes place is one in which they appear to lengthen out into slender cords,
the extremities of which soon begin to extend laterally, and the primitive
enamel organ is formed. Accompanying this lateral extension of the
periphery, a bell-shaped outline is assumed, which phenomenon is
16

242

ANATOMY.

rapidly increased by a specialization of the surrounding cellular tissue
forcing into the concavity.

This bell-shaped proliferation of cells, given off directly from the
tooth-band, to which it continues for a time attached, together with the
specialized connective tissue crowding into its concavity, constitute the
tooth-germs, the former being the enamel organ, and the latter the dentin
organ. It will, therefore, be seen that the enamel is dependent upon
the oral epithelium for its development (ecdermic), while the dentin
springs from an entirely different source — the connective tissue of the jaw

Epithelium of
Upper Jaw

Infant Layer
of Cells

Epithelium of Lower Jaw

Neck of Cord

Stellate Retic-
ulum

Ameoblasts

Dentin Papilla

Forming Be

Fig. 1S4. — Developing Tooth-germ, Twelfth Week. X 4c.

(endermic). The enamel organ rapidly undergoes a cellular transfor-
mation: its concavity is increased, and the bell-shaped outline more
strongly defined. Accompanying this change in form it gradually recedes
from the surface, and its connection with the tooth-band becomes less
secure. The connective-tissue cells, which have been rapidly filling in
the concavity of the enamel organ, are also preparing to take upon them-
selves a special function, that of the formation of the dentin. Up to this
period (tenth week) the enamel and dentin germs are not definitely
separated from the surrounding cellular structure, but now a gradual
transformation takes place, whereby the tooth-germs become enveloped
in a sac-like covering — the dental follicle.

DEVELOPMENT OF THE TEETH. 243

Enamel Organ (Figs. 183, 184, and 185). — This portion of the
tooth-germ, as previously stated, is derived from the concave or lingual
surface of the tooth-band, which in turn is derived from the surface
epithelium. From the free extremity of its slender cord-like attachment
it spreads out and forms a hood-like covering to the dentin germ. The
surface of the organ contiguous to the dentin germ, or dentin papilla, as
it is frequently called, is concave in the direction of the oral surface,
being thickest over the center of its concavity, thinning down as its per-
iphery is approached. Externally, the enamel organ is covered by an
epithelial layer, which is reflected upon its inner surface or that in contact
with the dentin papilla. These two layers are named according to their
location, the external and internal epithelium of the enamel organ. Placed
between these two layers, and constituting the bulk of the organ, are
numerous stellate bodies which penetrate a layer of rounded cells, the
stratum intermedium, and finally reach the internal epithelial layer
known as the enamel cells or ameloblasls. It is from this internal layer
of epithelial cells that the enamel is calcified, and they are, therefore,
the essential cells of the enamel organ. In the fully developed enamel
organ, there are to be found, therefore, four distinct layers of cells, the
external epithelium, stellate reticulum, stratum intermedium, and internal
epithelium, or ameloblasls. As its name implies, the function of the
enamel organ is principally that of enamel calcification, but in the opinion
of many writers its primary activity is that of molding the tooth-form
as represented by the dentin papilla, and it is not until this latter organ
has assumed the form and extent of the future tooth-crown that dentin
calcification begins.

The life of the enamel organ may properly be considered as begin-
ning when the bulbous extremity of the specialized cells given off from
the lingual face of the tooth-band become invaginated, and from this by
a rapid proliferation of its cells it passes on by successive stages assum-
ing the various forms common to it. This proliferation and differentia-
tion of cells continues up to the time of beginning of calcification, but
with the advent of this phenomenon certain parts of the organ begin to
degenerate. This degeneration may or may not be classed as an atrophy
of the cells interested, but the fact that a new tissue is generating, and
gradually occupying the space previously taken up by the formative cells,
calls forth a demand for the removal of the latter by the former. The
cells which first undergo this change are those of the internal epithelium
and stratum intermedium, the individuality of these two layers evidently
being kept up by migratory cells from the stellate reticulum. It is argued

244

ANATOMY.

by some writers that the external epithelium begins to atrophy at this
period; by others this change is not recorded until the enamel cuticle has
been deposited to effectually seal the young tissue and protect it until well
desiccated. While there appears to be a decided disposition upon the
part of this outer layer of cells to change, they do not disappear, and the
alteration is not one which affects the shape of the cells, for they remain
flattened or prismatic with their long axis placed parallel with the anlage
of the crown. The stellate cells making up the bulk of the organ are,

FlG. 185. — Developing Tooth-germs, Enamel Organ, and Dentin Papilla.

in common with those which inclose them, continually undergoing a
degenerative change, at least this is true of those cells closely associated
with the stratum intermedium, for in this location they rapidly proliferate,
shed their many processes, and gradually take on the characteristics
common to this layer of which they eventually become a part. A careful
examination of no less than one hundred enamel organs in all stages of
development, and by section cut transversely, obliquely, longitudinally,
etc., fully justifies the statement that the real life of the enamel organ
begins as previously stated, and continues until the structural arrangement
of the enamel is completed.

DEVELOPMENT OF THE TEETH.

245

The question of form in the enamel organ — that is, its external epithe-
lium — is one which may be advantageously used in the consideration of
the life and function of its different cell layers. It has been said that the
apparently extravagant area taken up by the enamel organ subserves
the purpose of reserving space for the growing tooth-crown, but there are
many reasons why this theory cannot be accepted. In the first place,
the extent of the organ or the space existing between the dentin papilla
and the outer enamel epithelium does not in very many instances corre-

Fig. 186. — Developing Tooth-germs. Longitudinal Sections from Buccal to Lingual.

spond to the bulk of enamel when this tissue is completed at a given point.
In the developed tooth we find the enamel thickest over the cutting-edges
of the anterior teeth and about the summits of the cusps of the cuspidate
teeth, while these same parts are represented during the cellular stage of
development by the external layer of cells closely associated with the
surface of the papilla.

Again, in very many instances the outline of the tooth is definitely
represented by the cells making up the dentin papilla (Figs. 185 and 186),
but the surrounding epithelial cells are characterized by an unbroken

246

ANATOMY.

semicircular margin describing the extent and form of the enamel organ.
Exception may be taken to this hypothesis from the standpoint of genera-
tive changes, and these in a great measure have much to do with the
relative outlines assumed by the two organs, but by studying very many
sections representing nearly every stage of the process, and all of them in a
measure showing the same characteristics, nothing but a definite opinion
can result.

Of the many changes in general form which the enamel organ
undergoes, none are so pronounced and positive in character as those

Fig. 187. — Developing Tooth-germs. Longitudinal Section from Mesial to Distal.

described by the inner tunic, and first recorded when the bulbous end of
the specialized cells becomes invaginated by the mesodermic connective-
tissue cells forcing themselves into it. This is an alteration which is
gradual and continuous up to the time of beginning of calcification, and
while the cells forming the dentin papilla are generally accorded the power
of "pushing" or "forcing" their way into those derived from the epi-
blast, the latter has always been recognized as having a controlling influ-
ence over the former. In this connection a reasonable doubt presents

DEVELOPMENT OF THE TEETH.

247

itself covering the theory so long accepted that the early function of the
enamel organ is one which in a measure superintends the contouring f
the tooth-crown as first represented in the dentin papilla. When the
character of the two embryonal tissues making up the two germs is Com-
pared, we find the dentin germ possessing all the characteristics favor-
able to a rapid proliferation of its cells resulting in a highly vascular
compact tissue. On the other hand, the bulk of the enamel organ j s a
gelatinous-like mass, one that would readily succumb to the pre ssure

Fig. 188. — Developing Tooth-germs in Transverse Section.
A, Stellate reticulum. B, Papilla. C, Cartilage cells.

exerted by those active connective-tissue cells within its borders. When
thus considered, the evidence is almost sufficiently convincing to reverse
the generally accepted theory, placing the general form of the enamel
organ as subservient to the dentin papilla.

Figures 185, 186, and 187 illustrate some of the variations common to
the general form of the enamel organ, and afford a good idea of the rela-
tionship existing between the enamel organ and the dentin papilla, in teeth
both of the simple and complex class. These were taken from sections

248

ANATOMY.

which represent a period just prior to the generation of the ameloblasts
and odontoblasts, at which time the external and internal epithelial
layers of the enamel organ most closely resemble one another, in general
outline. It is from this aspect and from sections cut longitudinally that
most of the information given by the older writers has been derived.
Few attempts have been made to show this organ in sections transverse
to the long axis of the tooth anlage. In figure 188 the germs of two teeth
are shown by a section made in this direction. One striking feature here

$**.

Fig

A. Inner tunic of enamel organ. H, Cells <>f dentin papilla.

illustrated is the relationship existing between the inner and the outer
tunic of the enamel organ, and attention is called to the apparent coales-
cence of these two layers at those points which represent the mesial and
distal surfaces of the developing crowns. This condition is apparently
brought about by the cartilage cells forcing the peripheral cells of the
enamel organ into direct contact with the inner tunic completely obliterat-
ing the stellate reticulum in these localities. As a result of this lateral
pressure the outer epithelial cells representing the labial and lingual

DEVELOPMENT OF THE TEETH.

249

surfaces have become widely separated, but with no perceptible alter-
ation in the character of the cells composing the stellate reticulum. The
relationship existing between the tooth-germs and the surrounding parts
is one that will continue throughout the generation of the organs, and
makes questionable the theory that the stellate reticulum performs
the function of reinforcing or providing the ameloblasts with nutrient or
calcific material. If these same germs were examined, as they usually
are, in longitudinal section (see Figs. 185, 186, and 187), the investigator

S>f

|#.

Fig. 190.
A, Cells of Dentin papilla. B, Elongated cells of inner tunic.

would at once arrive at the conclusion that there was an equal distribu-
tion of the stellate cells about all sides of the dentin papilla. In their
very early life they apparently establish an equal bulk about all sides of
the dentin germ, but with the preparation for the growth of the alveolar
walls they may assume the proportions shown in figure 188. At a period
corresponding to the complete envelopment of the dental germs by the
dental follicle the development of the buccal and lingual walls is well
under way, but as yet no provision has been made for the septa between

250 ANATOMY.

the teeth, and it is undoubtedly to the approach of this latter phenomenon
that a definite lateral pressure is brought to bear upon the approaching
of the follicles.

Now let us pass to a consideration of some of the characteristics of
the various cell layers composing the enamel organ. These are desig-
nated according to their location, so far as three of the four layers are
concerned; in fact, the remaining cells, or those which receive their name
from their form, can scarcely be classified as a distinct layer, these stel-
late cells not being of uniform thickness in all parts of the organ. The
first layer of cells, or those making up the inner tunic, will be traced from
their primary spherical condition to their final generation into ameloblasts.
Figure 189 shows the character of these cells at a very early period, corre-
sponding to the sixteenth week in the human fetus. They are for the
most part spherical or slightly oblong multinucleated cells, and are more
or less closely associated. They partake very much of the nature of the
connective-tissue cells surrounding them, being differentiated from these
principally by a transparent zone not unlike the specialized matrix im-
mediately surrounding cartilage cells. About the first change recorded
in these cells (see Fig. 190) is one in which they become markedly elon-
gated or cylindrical, but during this process of differentiation some of the
cells apparently recede, while others advance in the direction of the
papilla, lining up in a single layer to become the early enamel cells, the
cells which have been thus forced to the rear subsequently developing
into ameloblasts as the older cells atrophy. At this period the stratum
intermedium also asserts itself in the form of a distinct layer of rounded
cells, to be described later on. When first observed, these cylindrical
cells are devoid of processes, but are provided with rounded extremities,
with little or no variation between the end directed toward the papilla
and that looking in the opposite direction. This form is one which
persists in all of the cells included in this layer until a definite body of
cells is formed contiguous to the dentin papilla, these latter cells becoming
more markedly elongated and further differentiated by the addition of
processes, while the remaining cells, or those nearest the stratum inter-
medium, continue for a time unchanged. The next alteration in the
character of the inner tunic is one well illustrated in figure 190, in which
the body of the generating ameloblastic cells rapidly recede from the surface
of the papilla, while the elongating processes reach out to this latter point,
all of this occurring before the appearance of the odontoblasts. Soon
after this latter change in the cells of this layer the odontoblasts are
developed and form the periphery of the dentinal tissue. All of these

DEVELOPMENT OF THE TEETH.

25 1

changes are of course first recorded about the free extremity of the tooth-
crown, becoming less noticeable as the union of the outer and inner tunics
is approached.

A study into the special characteristics of the fully developed amelo-
blasts shows that these active cells are the result of a gradual change in
the character of the columnar epithelia common to both the external and
internal epithelial layers in the primitive enamel organ.

Fig. 191.

A, Stellate reticulum. B, Stratum intermedium. C, Ameloblasts. D, Forming enamel.
E, Calcified dentin. F, Odontoblasts.

Next in importance to the internal epithelial layer are those closely
associated cells making up the stratum intermedium (Fig. 192). Primarily
oval or spheroidal in form, we find these cells gradually assuming a
columnar outline and occupying a position parallel to the long axis of the
crown. It may be said that the general character of these cells is inter-
mediate between those destined to become the proper enamel cells and

252

ANATOMY.

those stellate cells making up the bulk of the organ. There appears to
be much confusion, at least considerable doubt, in regard to the office of
the cells of the stratum intermedium. It is most likely that these cells are
not only intermediate in character, but are also intermediate in func-
tion to those cells upon either side of them, recruiting the ameloblastsas
they fall, while in turn they themselves are supplied with nutriment from
the enamel pulp or stellate reticulum. No stronger proof that these
cells are secondary in importance to the ameloblasts need be mentioned

Fig. 192.
\, Generating ameloblasts. B, Rounded cells of stratum intermedium. C, Stellate reticulum,

than reference to the fact that they are always more generously supplied
to those parts about to undergo calcification. Nor is their increase in
numbers the only reason for believing that they are thus employed, for
at the same time those cells most closely associated with the developing
ameloblasts take upon themselves a decided change in outline. This
alteration may be brought about by the conditions which influence the
shapes of all cells, i.e., by the pressure of surrounding cells or by
their preparation for functional activity, or both. Some of the older

DEVELOPMENT OF THE TEETH. 253

writers speak of the cells of this layer as being branched, and in this
way closely resembling those of the stellate reticulum. By strong am-
plification it is somewhat difficult to distinguish between the two
layers, but most certainly if there are branched cells present they are con-
fined to the intermediate zone, and should properly be classed with
those of the stellate reticulum.

The cells of this layer do not long remain columnar with a general
direction at right angles to the forming ameloblasts, but as these latter
cells they become spheroidal and extremely closely associated in the deeper
portion of the layer; in fact, cells corresponding to these in general appear-
ance may be found in connection with the fully developed ameloblasts,
being observed to best advantage by the aid of a high power objective
and a full flood of light from a powerful sub-stage condenser. The
cells thus found appear to be distributed at regular intervals about the
ameloblastic layer, and are so closely allied to the cells of the stratum
intermedium that they may be considered as migratory cells from this
layer. In the earlier stages there appears to be no definite line of demar-
cation between the cells of the inner tunic, and those composing the
stratum intermedium, but soon after the establishment of the amelo-
blasts the two layers are strongly differentiated by the interposition
of a highly transparent membrane covering the outer extremities of the
ameloblasts. After they are thus definitely separated from the enamel-
forming cells, a most radical change takes place in their character; they be-
come markedly elongated, and by anastomosing form a series of continuous
chain-like belts about the ameloblastic layer, the number and further char-
acter of which are dependent upon the extent to which the ameloblasts
have performed their function. If at any time there is a similarity between
the cells of the stratum intermedium and the stellate reticulum it is at
this period, because the former cells begin to lose their individuality,
although under low power they still appear as a distinct layer (Fig. 191) .

There is probably no body of cells directly interested in the develop-
ment of the tooth tissues so widely discussed as those making up the so-
called stellate reticulum, and while the chief basis for argument has been
with reference to their function, the general character and form of the
cells have received but little consideration. Ever since the first description
of this portion of the enamel organ the cells therein have been charac-
terized as "star-shaped," and while this stellate form is the most common,
it is by no means a universal condition. The form of the cells in common
with the other cells composing the organ appears to be much influenced by
the position which they occupy, and by the age of the organ, those cells in

254 ANATOMY.

the region of the inner tunic partaking of the globular form characteristic
of this layer (see C, Fig. 192), while those closely associated with the outer
tunic are inclined to be columnar or somewhat elongated. While the
bodies of the cells in these respective locations are more or less influenced
by their environments, they still retain to a certain extent the stellate
feature by their many processes. But it is in the center of this myxom-
atous epithelial product that the most perfect stellate cells are located.
We find, therefore, where this part of the organ is of the greatest width,
that the true stellate cells are the most numerous, while at the summit of
the crown and at the base of the organ, at both of which points the outer
and inner epithelial layers are closely associated, the star-shaped cells
are almost wanting. In the study of this layer very much depends upon
the thickness of the section, only the thinnest possible sections affording
an opportunity for a correct conception. This is, of course, true of all
parts of the organ, but the peculiar character of the stellate reticulum
makes it especially necessary that great care be bestowed upon the prep-
aration of the section. In transverse section the cells present no charac-
teristic differences from those shown when the section is made longitudi-
nally. One very pronounced feature about the cells of the stellate retic-
ulum is the granular appearance of their protoplasm, resembling very
closely the flattened squamous cells from the epithelium of the mouth,
and it is no doubt this special feature which furnishes the ground for the
opinion of many writers that it is a peculiarly modified epithelium. One
peculiarity in connection with this tissue which iscontrary to the generally
accepted character of epithelial cells is the abundance of intercellular
cement substance; but when the many minute spines or processes
are considered as a part of the individual cell, the proportionate
quantity of cellular and intercellular substance is somewhat decreased.
The connecting processes are quite similar to those described by Stohr
as connecting bridges of protoplasm, while the cells themselves may be
otherwise described as prickle-cells. The change in the form of the cells
of this layer is not due to the presence of neighboring cells, as in the case
with most epithelial cells, but, being soft and extremely plastic, it is more
than likely that their form is strongly influenced by the tension of their
connecting filaments. One of the most marked alterations in the
general character of this part of the enamel organ is that which takes
place at a time corresponding to the beginning of amelification, and is no
doubt attributable to this phenomena. The cells which up to this period
have remained widely separated now become more closely associated,
not so much by a change of position as by what appears to be an increase

DEVELOPMENT OF THE TEETH. 255

in the size of the cell body with a corresponding decrease in the length
of the anastomosing processes.

It is a fact admitted by most histologists that the peculiar star-like
nature of the cells of the stellate reticulum is one principally brought
about by postmortem changes, and that in reality they are polygonal cells
tilling up a greater part of the tissue with but little intercellular sub-
stance. That some shrinkage and distortion does take place may be
proved by the examination of a section which has accidentally or other-
wise become for a moment dry during its preparation, in which case
little can be seen but the connecting processes, and even these are much
shrunken. All the cells contained within the organ are more or less
affected by this procedure, but none of them exhibit such a marked
change in the outline as those of the stellate reticulum.

The layer of cells which is usually considered of least impor-
tance is that which makes up the outer tunic. In the young enamel
organ the cells partake very much of the nature of these forming the inner
tunic, but the older the organ becomes, the more dissimilar are the two
layers. Primarily this layer is constructed of a single row of elongated
cells, placed with remarkable regularity, upon the inner side of which are
a number of similarly formed cells variously disposed, but with a common
direction at right angles to those previously referred to. Like the inter-
nal epithelial layer, the cells of the outer tunic partake more or less of
the nature of the stellate cells in passing from the single row of well-
defined cells toward the stellate reticulum. While in the beginning
the external epithelial layer is strongly differentiated from the sur-
rounding cells, this is of but short duration. The atrophy of
this layer begins with the appearance of the fully developed amelo-
blasts, by which the regular arrangement of the cells is greatly disturbed
by an apparent breaking up of the entire layer. Many reputable writers
claim that the external epithelial layer is of little or no interest, save, as
Tomes puts it, "as a matter of controversy." This admission upon the
part of so eminent an authority practically opens up a new field for re-
search, especially so when we consider that various other writers (Wald-
eyer, Kolliger, and Magitot)have expressed conflicting opinions in regard
to it. After carefully following the changes which occur in this layer
from its earliest inception up to an advanced stage of calcification, it
would appear that while marked changes occur in the character of the
individual cells as well as in the general appearance of the layer, it is
nevertheless persistive, and in some way is essential to the process of
amelification, even to a more marked degree than are the cells of the

256 ANATOMY.

stellate reticulum. One important reason for this belief is based upon
the fact that in the cuspidate teeth there appears at a time corresponding
to the beginning of amelification, a decided disposition in the cells of
this layer to dip down and completely divide the stellate reticulum be-
tween the forming cusps. That this alteration is one instrumental or
essential to the calcifying process receives additional proof by referring
to figure 185, which shows the fully developed enamel organ with the

Dental
Ridge,
Upper
^ Jaw

Oral Cavity

Tongue —

^g Developing
\ Tooth-germ

Fig. 193. — Section through the Floor of the Mouth of Human Fmbryo.
Twelfth Week. X 30.

exception of the actual appearance of the ameloblasts, and the lack
of any attempt upon the part of the external epithelial layer to penetrate
between the cusps.

Dentin Organ (Fig. 184). — This part of the tooth-germ, formed from
the connective tissue of the primitive jaw, occupies the concavity of the
enamel organ, and at an early period begins to assume the form of the
future tooth-crown. Thus, primarily, the papillae for the incisors will
have their cutting-edges outlined by three small lobes, each of which
represents a separate point of calcification, while the papillae for the molars
will be molded according to the number of cusps of the future tooth, a
small tubercle making its appearance for each cusp. In its inception the
dentin papilla is composed of cellular tissue identical with that of the sur-

DEVELOPMENT OF THE TEETH. 257

rounding parts. The growth of the papilla is in the direction of the surface ;
at the same time the enamel organ forces itself more deeply into the sub-
stance of the parts, not only overhanging the coronal extremity of the
papilla, but extending about and inclosing its lateral walls. Accompany-
ing the growth of the papilla is a rapid change in its structure, becoming
more vascular throughout, and its peripheral cells, differentiating, form
the essential dentin cells — the odontoblasts. This layer of cells is in
close relation to the enamel cells of the enamel organ, the combined
activity of the two finally resulting in the calcification of the tooth-crown.
The dentin papilla, which eventually becomes the tooth-pulp, decreases
in size as calcification proceeds to the dentin, all additions to the calci-
fying surface taking place from within; while the enamel organ may be
said to increase in size, the calcific action in the enamel progressing from
within outward.

The Cells of the Dentin Papilla. — In the early life of the dentin
germ, the cells are all simple embryonal connective-tissue cells. After
differentiation takes place they are widely scattered and are of four
varieties; spindle-shaped, round, stellate, and the elongated or club-
shaped odontoblasts. None of these are constant in location except the
layer of odontoblasts, which, as has been said, are arranged in a single
row on the surface of the periphery of the organ, this zone being classed
by the older writers as the membrana eboris. Like the ameloblasts of
the enamel organ, the odontoblasts do not make their appearance until
the papilla has assumed certain proportions, this about corresponding
to the size and form of the dentin of the future tooth. Immediately
beneath the layer of odontoblasts appears a zone almost devoid of cells.
This is followed by a district in which the cells are quite numerous, and
finally when the central portion of the papilla is reached the cells are again
few in number and widely scattered. For the most part the cells in the
interior of the papilla are spindle-shaped or stellate, having rounded
nuclei about which there is a small amount of protoplasm which pene-
trates the intercellular substance by numerous hair-like processes.

The Odontoblasts. — These are club-shaped or flask-shaped cells,
each provided with a large nucleus which usually assumes the outline of
the enlarged end of the cell which is directed toward the interior of the
papilla. From the opposite end of the cell, or that directed toward the
enamel organ, and in close proximity to its concave surface, one or more
protoplasmic processes are given off. These persist and are finally encap-
suled within the calcified dentin, forming the dentinal fibers. These
cells are very closely associated, so much so, in fact, that their enlarged
1 7

258 ANATOMY.

extremities are almost or quite in actual contact, more or less space ex-
isting between the constricted portion of the cells as they pass toward
the surface.

Germs for Permanent Molars. — Reference has been made to the
fact that the enamel organs for the deciduous teeth are given off from the
tooth-band at a point somewhat distant from its free margin, so that the
tooth-band is continued beyond the primitive enamel germ, this free
margin of the band afterward generating the enamel organ for the succe-
daneous tooth. As the twelve permanent molars are not succedaneous
teeth, some other means must be provided for their development.

Opinions of various writers upon this subject are somewhat conflict-
ing. The theory is advanced by some that as the jaw increases in length
the tooth-band and lamina primarily provided for the deciduous teeth
are extended backward, first giving off a bud for the first permanent
molar; at a somewhat later period, and with the increase in the growth of
the jaw, an additional bud is generated for the second molar, the third molar
being provided for in a like manner. Another theory, and one generally
accepted as correct, is that the cords for the permanent molars spring
individually and directly from the subepithelium. There may be found
an exception to this in the case of the first permanent molar, which some-
times appears to have its origin from the distal follicular wall of the second
deciduous molar. Whatever theory be accepted in regard to the genesis
of these permanent organs, the process of development after the appear-
ance of the primary bulb or enamel germ is identical with that of the de-
ciduous teeth.

The Dental Follicle, or Tooth-sac. — During the early life of the
tooth-germ, both the enamel organ and the dentin papilla are differen-
tiated from the surrounding parts by dissimilarity of structure only, but
as development proceeds, a more definite separation appears between the
tooth-generating organs and the general tissues of the primitive jaw,
this separating medium being known as the dental follicle. The term
"follicle" is only one of a number applied to these parts, "dental saccu-
lus," "tooth-sac," and other appellations being employed with equal
significance. By some writers it is customary to apply the term " follicle "
up to the period of complete closure, the term "sac" or "sacculus"
being employed after that time. There appears, however, to be little
foundation for such a distinction, the terms being synonymous. Again,
the follicle is frequently referred to as meaning the sac and its contents,
but this usage is a misapplication of the term. There appears to be no
well-founded reason why the follicular wall or sac should not be referred

DEVELOPMENT OF THE TEETH.

259

to as such, regardless of the formative organs within. As to the develop-
ment of the tooth-follicle, it appears to be a generally accepted theory
that at a very early period there is developed from the base of the papilla
cells which, differentiating, form the walls of the follicle. By this growth
of cells the periphery of the papilla is first surrounded, and this step is
soon followed by an extension of the cellular structure in the direction
of the surface epithelium, to the deep layer of which the cells become
firmly attached, and in so doing inclose the enamel organ, which hangs like
a hood over the extremities of the papilla. The tissue thus formed from

Calcified Dentin

Dentin Papilla,
or Tooth-pulp

Wall of Follicle

Developing Bone

Wall of Follicle

Developing
Bone

Fig. 194. — Developing Tooth about the Fourth Fetal Month. Appearance of

the Tooth-follicle.

the base of the dentin germ is continuous with and similar in its origin to
the pulp-substance. The primitive tooth-germ, during the formation of
the follicular wall, is found swinging in a membranous pocket, being
supported by the epithelial band, which, in turn, is attached to the oral
epithelium; but as the walls increase and completely inclose the germs,
which is accomplished about the fourth fetal month, the epithelial band
is broken and the second or saccular stage of tooth-development is reached.
The walls of the follicles are made up of two layers; the outer layer is
dense and firm, and finally becomes the dental periosteum; the inner
layer is thin, frail, and in the recent state somewhat transparent, and at
an advanced period assists in the formation of the cementum.

260

ANATOMY.

Having thus briefly described the primary stage of tooth-develop-
ment, the careful study of which can only be pursued with the aid of the
microscope, we will now pass to the secondary or saccular stage. By
the introduction of a number of illustrations, prepared from original
dissections by the author, this phase of the subject will be readily
comprehended.

Before continuing the subject of tooth-development, it will be
eminently proper to briefly describe those parts directly concerned in the
process. At a very early period of fetal life we find preparations are being

Calcified Dentin
Enamel

Outer Enamel

Epithelium

Epithelium of
Jaw

Dental Ridge

Germ for Perma-
nent Incisor

Dentin Papilla

l'n;. 195. — Development of De< iduous Im isnr, from Human Fetus.-

(Afler Cyst.)

made for the development of the maxillary bones. That these are about
the first bones to be called into functional activity accounts in a measure
for their very early development. The osteoblastic activity in the inter-
cellular substance destined to become the inferior maxilla begins about
the middle of the second month of fetal life, while at a somewhat later
period a similar action takes place in the region of the superior maxilla. A
detailed description of the body of these bones having been given on another
page, it will not be repeated here, but to that portion which gives lodg-
ment to the tooth-germs, and which in a measure is controlled by their
presence, some attention must be given, and for this purpose the mandi-
ble will principally be used.

DEVELOPMENT OF THE TEETH.

26l

Fig. 196 represents the lingual face of the lower jaw after removal
from a three months' fetus. Attached to it is the remaining portion of
Meckel's cartilage, which by this time is much wasted. It will be re-
called that this cartilaginous band appears in the mandibular processes
before the beginning of the second fetal month, being formed in two
distinct halves, the free ends of which finally unite at the median line,
forming a continuous support or framework, about which ossification
takes place. At a corresponding period, and in a similar manner, two
like processes are thrown out for the superior maxilke, but, unlike
Meckel's cartilage, these do not unite at the median line, but stop short
of this, the space thus resulting being provided for by two additional

That Part of Cartilage not yet Ossified.

Head of Malleus
Cartilage of Incus

Handle of Malleus

1
)

Ossified Mandible
Fig. 196. — Developing Mandible, Three-month Fetus.

processes, which shoot down from the region of the forehead and provide
for the development of the intermaxillary bones. About the middle of
the second month a center of ossification appears in the neighborhood
of the future mental foramen, quickly followed by others at the symphy-
sis and at the angle. These secondary centers soon unite with the
primary one, and by the end of the second fetal month the osseous con-
tour of the primitive jaw is established. While ossification takes place
in the membrane surrounding Meckel's cartilage, the cartilage itself
does not appear to be directly concerned in the process, and by the sixth
or seventh fetal month the mandibular portion completely disappears,
while that portion near the tympanum is ossified into the malleus. That
portion of the bone which forms above Meckel's cartilage and the inferior
dental nerve is that which finally gives support to the tooth-germs. This
cartilage is not confined to the human species, but is the common herit-
age of reptiles, rodents, birds, and fishes, in all of which it gives support
to the developing lower jaw.

262

ANATOMY.

Figure 197 represents the evolution of the mandible from the middle
of the third fetal month to the time of birth. It will be observed that
during this interval there is a gradual increase in the size of the bone,
but little alteration in its contour. By a constant and gradual osseous

deposit about the distal extremity of
the bone its length is increased to accom-
modate the additional teeth as they make
their appearance. While the external
form of the bone shows but slight
variation during this period, the internal
structure, or that wherein the tooth-
germs lie, is undergoing a complete
transformation.

Figure 198 is illustrative of these
changes; beginning with a simple groove,
or gutter, into which the tooth-follicles
hang, the follicles exerting a controlling
influence over its form. Next comes the
appearance of septa between the anterior follicles, which at this period
are somewhat irregularly placed in the arch, followed in a few weeks by
a well-defined partition between the cuspids and molars, until finally, at
birth, each follicle is inclosed in its individual crypt, with the single
exception of the second molar,
in which the distal septum, or
that which is to separate it
from the permanent first
molar, has not yet made its
appearance. As the tooth-

FlG. 197. — Evolution of the Mandi-
ble from the Third Fetal Month to
Birth, Two-thirds Actual Size.

follicles increase in size, by
the development of the teeth
within, they become more per-
fectly inclosed in the bony
vaults, the sides of the
alveolar walls arching over
and almost completely in-
closing the developing teeth.
Figure 199 shows the lower jaw of a seven-months-old child
embodying the condition above referred to. No sooner have
the crypts grown to this extent, than the resorptive action pro-
duced by, or provided for, the advancing crowns speedily results in

Fig. 198. — Evolution of the Mandible from Third
Fetal Month to Third Month after Birth.

DEVELOPMENT OF THE TEETH.

263

their downfall, to be again built up with the evolution of the permanent
teeth.

About the first visible sign of preparation for the development of the
teeth, other than that made apparent by dissection, may be observed as

Mucous Membrane and Periosteum Lifted Up

Fig. 199. — Inferior Maxilla of Seven-months-old Child.

early as the beginning of the third fetal month, when, upon opening the
cavity of the mouth and looking upon the palate, a well-defined infold-
ing of the epithelial eminence will be seen. In figure 200 this condition
is shown by a dissection through the oral cavity of a four months' fetus,
the outer fold being that of the cheeks and lips, while within are the hard

Labiodental Space

Labial Fold

Primitive Dental
Groove, so called

Fig. 200. — Section Through the Mouth of Four-month Fetus.

palate and primitive alveolar ridge. The mouth at this period has
passed the rudimentary state, the transverse plates which contribute to
the formation of the hard palate. having approached each other until the
oral and nasal cavities, heretofore existing as a single buccal cavity,
have become separate and distinct. The infolding of the oral epithelium,

264

ANATOMY.

Maxilla

as outlined on the summit of the primitive alveolar ridge — the primitive
dental furrow so called — marks the position of the tooth-band, from
which are given off the incipient tooth-bulbs. For the purpose of further
investigation, a dissection of these parts was made and the maxillary
bones removed, after which they were divested of their fibrous covering,
including the periosteum. That portion which overlies the palatal
processes was readily lifted in one sheet, while that upon the facial surface

was separated at the median line and
stripped independently of the other
(Fig. 201). The removal of these
tissues is readily accomplished until
the margins of the partly formed
alveoli are reached. Here the peri-
osteum dips down into the various
crypts, and serves as a lining mem-
brane for them, and probably con-
tributes fibers to the outer layer of
the follicular walls. After advancing
thus far, the detached tissues may

FIG. 201. — Superior hetal Maxilla,
showing manner of Dissection to E \ po se be grasped, and by careful manipula-

Tooth-sacs. tion the tooth-follicles containing the

formative organs removed from their respective vaults and turned over for
examination.

Figure 202 shows the result of such a dissection. On the left, the
palatal plates and alveolar walls of the divested bones may be observed;
on the right, is the fibrous covering, which has been turned completely
over after removal from the bones, having firmly attached to it the
ten tooth-follicles for the deciduous teeth. If this dissection be made
without the precaution of lifting the periosteum, the follicles would not
cling to the oral membrane with sufficient tenacity to permit of their
ready removal. It may be of some interest to note that at this early
period the position of the follicles containing the germs for the lateral
incisors is that which the tooth is forced to occupy up to and frerpuently
beyond the eruptive period, being crowded within the tooth-line by the
central and cuspid follicles, in consequence of which the lateral crypts
are thrown well into the palatal plates.

Figure 203 represents the sacs broken down, exposing to view the
dentin papillae, or those structures destined to become the tooth-pulps.
The position which these occupy in the illustration is exactly the reverse
from that which they assume when in position in the follicle, being thus

DEVELOPMENT OF THE TEETH.

265

reversed that a better idea of their shape may be obtained. Prior to the
twelfth or thirteenth week of fetal life this incipient bulb or papilla is
without definite form; but by the latter period each papilla begins to
assume the contour of the future tooth-crown, as faintly outlined in the
illustration, those of the incisors presenting the angular form of the

Tooth-follicles

Maxilla

Oral Mucous Membrane

Fig. 202. — Dissection upon Superior Maxilla, Fourth Fetal Month, Exposing

Tooth-follicles.

future cutting-edge, the cuspids that of the single cone, while the coronal
extremities of the molars are represented by outlines corresponding to
the future cusps and marginal ridges. Besides the dentin papilla, there
is contained within the follicular walls the organ which later on is pro-
ductive of the enamel, but up to this time has been actively engaged in

Dentin Papillae

Maxilla

Oral Mucous Membrane, Turned Completely Over

Fig. 203.

molding the tooth-form as outlined by the papilla. To return to the tooth-
follicle, the dissection in this instance being upon the lower jaw of a
four months' fetus. Figure 204 shows the mandible removed and the
dissection carried to the point at which the follicles may be lifted from
their bony encasements, this being accomplished by an incision along

2 66

ANATOMY.

the base of the bone, followed by a stripping of the membrane first from
the facial and then from the lingual side of the bone. When these two
flaps reach the margin of the crypts, they are firmly grasped and the
follicles removed from their sockets, as illustrated in figure 205.

At the beginning of the saccular stage of development the form of

Oral Mucous Membrane and Periosteum Dissected from Mandible

Attachment of Follicle to Oral Membrane (Enlarged One-third)
Fig. 204. — Manner of Dissection to Expose Tooth-sacs.

the future tooth-crown is well outlined by the dentin papilla, which in
figure 206 is brought into view by a dissection of the walls of the follicles
shown in figure 205 without breaking the attachment existing between the
two. As in the case of the follicle, much confusion of terms in regard to this
structure exists, the "dentin bulb," the "pulp," "dentin germ," and the

Tooth-follicles Oral Membrane

Follicle or Perma-
nent First Molar

Fig. 205. — Tooth-follicles Removed from Mandible, Fourth Fetal Month.

"papilla" being used ad libitum, without apparent regard for the struc-
tural changes which are continuously affecting the organ. It will, there-
fore, be proper to simplify this conglomeration of terms by a classification
appropriate to the various stages of development. The term "dentin
papilla" will best describe this part of the tooth-germ up to the time of

DEVELOPMENT OF THE TEETH.

267

beginning of calcification, subsequent to which the term "pulp" should
be employed. As previously stated, the enamel organ, until this period,
has been principally devoting its energies to the molding of the tooth-
form, and it is not until this model, as represented in the papilla, is com-
plete that the process of calcification begins. About the fourth fetal
month preparations for the calcification of the deciduous teeth are begun
by the development of the odontoblastic cells for the dentin, which first
made their appearance on the periphery of the dentin papilla, the summits
of the various cusps in the molars and the future cutting-edges of the
incisors being first affected. This phenomenon is soon followed by the

Space Ocupied by Oral Mucous
Enamel Organ Membrane

Follicular Wall

Dentin Papillze
Prior to Beginning
of Calcification

Fig. 206. — Tooth-follicles shown in Figure 201 Opened.

appearance of the ameloblastic cells for the enamel, which establish
themselves in the internal epithelial layer of the enamel organ.

Figure 207 is prepared from a dissection made in a manner similar
to that shown in figure 203, but at a period about a month later, being from
the superior maxilla of a five months' fetus. The dissection shows
the extent of calcification at this period, which process also defines the
position of the odontoblastic cells upon the extremity of the papillae. In
the incisors (one of which was lost in the preparation of the specimen)
the dentin may be seen capping the cutting-edges. The cuspids in this
subject have not yet begun to calcify, although it is not unusual to find
the cusp of this tooth receiving its lime-salts at this early period. In the
molars the summits of the various cusps, as well as a portion of the various
ridges descending therefrom, are undergoing the change produced by
the impregnation of the lime-salts. It is quite probable that these delicate
caps are at this time composed of dentin alone, the calcoglobulin which

2 68

ANATOMY.

precedes the enamel calcification forming somewhat later. From this
time forward the pulp undergoes a gradual transformation as to size and
form, and there is likewise a change in its cellular construction on those
parts adjacent to the calcific action. While the outline of the pulp is

Calcified Caps Tooth-pulp

Maxilla?

Oral Mucous

Membrane,

Turned Over

Fig. 207.

gradually changing, its original form is permanently recorded upon the
periphery of the dentin cap, which, when once formed, is immutable, all
additions taking place from within.

Figure 208 illustrates the result of a dissection upon the lower jaw
of the same subject, disclosing practically the same conditions, with the

Calcified Dentin Oral Mucous Membrane

Space Occupied by
Enamel Organ

Dentin Papilla?

Fig. 208.— Tooth-follicles Opened, Exposing Dentin Papilla; and Beginning of

Calcification, Fifth Fetal Month.

exception of the sac containing the developing cuspid, which was found
with a slightly calcified cap of dentin. This slight variation between the
development of the upper and lower teeth is one that is present in nearly
every instance, the latter being somewhat in advance of the former.

DEVELOPMENT OF THE TEETH.

269

In figure 209 the tooth-pulps, with their primitive cappings of dentin,
have been removed from the follicles, and a better opportunity of study-
ing the relations between the two parts is presented. By the conversion
of the coronal extremities of the dentin papilla into odontoblasts, and
their active calcification, some positive union between the two parts
might be expected. On the contrary, the dentine caps are readily

• +

Fig. 209.

removed, leaving the pulp beneath without the slightest rupture, so that
we find calcification is not a secretory or excretory metamorphosis, but
that the change takes place within the substance of the papilla itself,
whereby it is altered from an organic to an inorganic substance.

The next dissection was one upon the mandible of a six months'
fetus. Figure 210 shows the tooth-follicles removed from the partially
formed bony crypts in which they have been incased. x\t an early

Tooth-sacs of
Deciduous Teeth

Lingual Surface of Mandible
FlG. 210.

period of fetal life, and at a time prior to the completion of the tooth-
follicles, there is deposited beneath the tooth-germs a thin layer of bone,
which at once begins to assume the form of the partially developed fol-
licular walls. As the growth of the follicle proceeds, there is a correspond-
ing increase in the osseous deposit, the alveolar walls extending about
and accommodating themselves to the membranous sacs. Thus we find

270

ANATOMY.

in this portion of the maxillary bones a feature peculiar to itself — that
of a continuous transformation from its earliest inception to the adult
period, first developing about the temporary tooth-sacs and completely
incasing them, which is speedily followed by complete resorption of the
walls, again followed by a rebuilding during the evolution of the perma-
nent teeth, and again swept away with the loss of these organs. Figure 211

Follicular Wall Calcified_Caps Oral Mucous Membrane

Dentin Papilla
First Perma-
nent Molar

Tooth-pulps

Fig. 211.

illustrates the opposite side of the same jaw, with its outer or facial plate
removed, together with the intervening septa. The follicles are opened,
and the extent of calcification at this period (six months) made apparent.
The pulps and calcified caps are approximately as found when dissected,
save a slight settling of all the parts. The incisors have calcified to about
one-third their full coronal length; the unicusped .contour of the cuspid
has been established, as shown by the deposit of the lime-salts upon its

Fig. 212.

summit, and the upbuilding of the mesial and distal cutting-edges. The
first molar has about completed its occlusal surface, and, while the cusps
of the second molar are nearing completion, there is a lack of union in the
central and distal fossae. Immediately posterior to the second deciduous
molar, the sac containing the formative organs for the first permanent
molar is shown opened, exposing to view the dental papilla, which at

DEVELOPMENT OF THE TEETH.

271

this early period has assumed the form of the future tooth-crown, and
calcification is about to begin. Figure 212 illustrates the extent of calcifi-
cation in the deciduous teeth at the sixth fetal month. As the growth of
the teeth proceeds, it will be observed that the angularity which originally
accompanied the calcifying caps of the molars is gradually disappearing,
as is also the tritubercular form of the incisors and cuspids, this
change being brought about by the deposition of enamel to the parts.
Figures 213 and 214 show the calcified caps removed from the

Fig. 213.

pulps, and so arranged that both their external and internal anatomy
may be studied. Prior to this time there has been but little alteration in
the form of the dentin pulp, only a gradual decrease in its size being
noted; but now we find it being divested of many of its angles, particularly
those which originally served as a basal form for the coronal extremities
of the future tooth. With the disappearance of these the concavity within
the cap is slowly assuming the form of the future pulp-chamber.

That a better understanding of the saccular stage of tooth-develop-

Fig 214.

ment might be had, a transverse section was made through the molar folli-
cles, as shown in figure 215. In this the attachment of the follicular walls
to the deep epithelial layer is visible, while within the walls is the enamel
organ, the calcified dentin, and the tooth-pulp. As previously stated,
the enamel organ is seen suspended above and forming a hood-like invest-
ment to the calcifying structure. This organ not only overhangs the occlu-
sal surface of the tooth-crown, but completely envelops the sides of the
calcified cap and dentin pulp. Previous to the beginning of calcification

272

ANATOMY.

the enamel organ is in close proximity to the dentin papilla, the original
form of the latter being represented by the calcified dentin.

We have now arrived at that period of fetal existence when it is possi-
ble to study the macroscopic development of the permanent first molar.
Figure 216 represents a section of the lower jaw of a six months' fetus,

Enamel Organ Oral Mucous Membrane or Gum

Follicular Wall

Tooth-pulp Floor of Tooth-follicle
FlG. 215. — Dissection Showing Pulp, Calcified Cap, and Enamel Organ.

and displays not only the sacs of the deciduous teeth, but also that of
the permanent first molar. In most respects the evolution of this
tooth is similar to that of the temporary organs, having its origin from
the deep epithelial layer, either directly or by continuation of the tooth-
band backward. Preparations for its growth are begun as early as
the third fetal month, at which time the enamel organ is given off, and
oral Mucous Membrane thereafter the developmental process is

identical with that of the deciduous teeth.
There is one structure, however,
intimately connected with the develop-
ment of the permanent teeth not found
in connection with the deciduous organs
— the gubemaculum, or leading cord.
Figure 217 represents another section of
a six months' fetal mandible, with the
dentin papilla for the permanent first
molar turned out from the follicle after being rolled from its bony
incasement. Attached to the apex of the tooth-sac (which has been
turned back), and leading from it to the epithelium of the jaw, is the
gubemaculum. This fibrous structure was at one time thought to be
directly concerned in the development of the tooth. Although this is
denied at present, little is said in regard to its function, but it undoubtedly

Fig. 216.

DEVELOPMENT OF THE TEETH.

2 73

serves the purpose of directing the tooth to that position which it should
occupy in the jaw, and where the least resistance to its eruption is
formed by the foramen which the cord has established. Each of the
permanent teeth is provided with a similar membranous cord, an illus-
tration and more complete description of which will follow later on.

We have now arrived at a period when the subject under consideration

Oral Mucous Membrane Gubernaculum

2Ef^

Dentin Papilla
FlG. 217.

naturally becomes of deeper interest. I refer to that time when the being
changes from a complex dependent condition to one of self-providing inde-
pendence. Previous to the time of birth the teeth appear to be but little dis-
turbed by certain morbid conditions which might be present in the parent,
and from their earliest inception up to this period their development pro-
ceeds with but little interruption and with much regularity. Figure 218

I 1 4
ft i *

Fig. 218. — Deciduous Teeth at Birth. (Reflected picture.)

illustrates the condition of the deciduous teeth at birth; the central incisors
are calcified externally to the cervical line, the lateral incisors to a point cor-
responding to the summit of the palatocervical ridge; the cuspids have ad-
vanced somewhat beyond the angles of the crown, while the molars have
their crowns calcified to about one-half their completed length. With all
this progress as represented by the external contour of the tooth-crowns,
18

274

ANATOMY.

the internal form appears to be somewhat slow in assuming the shape of
the future pulp-chamber. From the beginning of the saccular stage of
development up to the time of birth there is but little increase in the
diameter of the tooth-sac, but there occurs a gradual increase in its length.
Figure 219 shows the mandible from a child one week old, with the greater
part of the external or facial surface of the bone removed, exposing

Oral Mucous Membrane

Fig. 219.

not only the sacs containing the developing deciduous teeth, but also
that of the permanent first molar. The relation of the sacs to the inferior
dental canal is apparent, as well as the firm attachment of the follicular
walls to the oral membrane. In figure 220 the tooth-sacs have been dis-
sected and the pulps removed from the calcified caps, presenting an
additional illustration of the amount of dentin deposit at this age. To

Calcified Caps

Dentin Papilla of
Permanent First
Molar

Pulps

Fig. 220.

further illustrate the size and form of the pulp as compared with the
calcified cap at birth, a transverse incision was made through the left supe-
rior maxilla, at a point corresponding to the base of the pulp, as shown
in figure 221. The calcified parts remain in position resting against the
remaining portion of the enamel organ, while the pulps are dislodged and
may be observed resting upon the incised surface. In this illustration

DEVELOPMENT OF THE TEETH.

275

the dentin papilla for the first permanent molar is also seen, being sup-
ported by the walls of the follicle, which in turn are attached to the oral

Calcified Caps, Resting against
Enamel Organ

Tooth-pulps Rolled Out

Section of Superior Maxilla

Pulp of Permanent First Molar

Fig. 221.

epithelium by the gubernaculum. It may also be noted that those parts
of the pulp corresponding to the cusps and marginal ridges show a decided
convergence of the surface toward the center.

Dentin Papilla for
Permanent First
Molar

Papilla

Calcified Cap

Walls of Tooth-
sacs

Maxilla

Oral Mucous
Membrane

Fig. 222.

Reference has been made to the formation of the follicular walls by a
differentiation of cells, which at an early period are given off from the

276

ANATOMY.

base of the papillae, and to the continuity of the two structures. Figure
222 was prepared for the purpose of showing these intimate relations.
The sides of the sacs were opened and turned back; the calcified tooth-
caps with pulps in position were grasped and given several revolutions,
thus twisting the remaining portion of the walls, the floor of which is
seen as a continuous structure given off from the pulp and connecting
it with the epithelium of the jaws.

PREPARATIONS FOR THE DEVELOPMENT OF THE
PERMANENT TEETH.

A little before the time of birth sufficient advance has been made in
the development of the permanent teeth to permit a study of their rela-
tions with the temporary organs. The early preparations for the growth
of these teeth was for a long time a subject of much controversy, some
writers advancing the theory that the buds for the permanent teeth were
produced or given off from the sacs of the temporary teeth ; others contend-

#•-,

Tooth-sac of Advancing

| |

MB ^^^k^

Deciduous Tooth

Gum

-v

*"^"^

Gubernaculum Tooth-sac o£ Receding Permanent Tooth
Fig. 223. — Section through Superior Maxillae, Sixth Month after Birth.

ing that the cords were derived from the remnants of the primitive cords
immediately after their rupture. The theory now generally accepted
is that the cord is given off from the primitive cord at a point in close
proximity to its attachment to the deciduous enamel organ. This can
only apply to those teeth which are succedaneous, and, therefore, does
not include the permanent molars, as heretofore stated. Whatever
theory be accepted in regard to the genesis of the permanent teeth, there
can be no mistake in regard to that part of the process which we are
permitted to ocularly investigate. I shall, therefore, proceed to describe

DEVELOPMENT OF THE PERMANENT TEETH.

2 77

the position and contents of the permanent tooth-sacs at birth.
Figure 223 presents the result of a vertical dissection through the su-
perior maxillary bones, the inner surface of the right maxilla being
exposed to view. Many of the frail processes, particularly those en-
tering into the construction of the nasal cavities, were lost during
the preparation of the specimen, the whole purpose of the dissection
being to show the sac of the permanent incisor and its relation-
ship to its predecessor. By this time calcification in the deciduous
incisor has so far advanced that the contour of the tooth-crown may be
plainly outlined through the walls of the sac. Resting against the palatal

Foramina for Gubernacula

,.._,„

jL^

m *'

* |L

•*^Bl_

*•» ^| ^Pv|

-. ■&

Fig.. 224.

concavity of the crown of the temporary incisor is the sac containing the
formative organs for its permanent successor. This permanent tooth-
sac does not long remain in such close proximity to the deciduous tooth-
crown, for as the latter advances toward the surface of the gum the former
recedes and is soon inclosed in a separate crypt, which, were it not for
the gubernacular foramen, would completely inclose it. Figure 224
represents a section of the left superior maxilla, introduced at this point
for the purpose of showing the position of the foramina for the gubernacula.
These may be observed immediately posterior to the incisor and
cuspid teeth. At birth these foramina do not exist as such, the partially
formed vaults containing the sacs for the permanent teeth appear-
ing as an extension of the temporary crypts in a palatal direction; but as the

278

ANATOMY.

temporary teeth advance and the permanent teeth recede, the roof of the
crypt is completed, and the foramen established by the presence of the
gubernaculum. This extension of the temporary crypts ts more clearly
demonstrated in figure 225, which represents one side of the lower jaw at
birth with the partially calcified deciduous teeth in position in the bone.

Tooth-sacs for Permanent Incisors Gum

Tooth-sac for Perma-
nent First Molar

Deciduous Teeth in Bony Crypts

Fig. 225.

Suspended above this is the gum, which has been dissected from the
bone, having attached to its under surface the sacs for the permanent
teeth. Those for the incisors are particularly well defined and their
place of lodgment in the bone readily noted. In the case of the cuspid,
both the deciduous tooth and the permanent tooth-sacs are in position
in the crypt. The cords which support the incisors are somewhat length-

Oral Mucous Membrane

Follicular Wall, Turned
Wrong Side Out

Papilla for Permanent First
Molar

Dentin Papilla? for Permanent Teeth, Upside Down

Fig. 226.

ened from the weight of the sacs, the gubernacula not assuming this
thread-like form until the permanent sacs have further receded.

Figure 226 shows the result of a dissection upon these permanent
tooth-follicles. The dentin papillae of the various teeth are seen in a
reversed position, with the follicular walls attached to their bases. At
this period the papillae for the permanent incisors may be compared to the

DEVELOPMENT OF THE PERMANENT TEETH.

2 79

tail of a fish, being perfectly transparent over its free extremity, which
feature is gradually lost as its thickened base is approached. The fish-
tail appearance is further represented by the division of the free extremity
into three distinct parts, each of which provides a separate point of calcifi-

Tooth-sacs of Permanent Teeth

Tooth-sacs of
Deciduous Teeth

Lingual Surface of Mandible

Fig. 227.

Tooth-sacs of Deciduous Teeth

Tooth-sac of Permanent Tooth

Periosteum and Mucous Mem-
brane from Hard Palate

Gubernaculum

Tooth-sacf or Permanent First
Molar

Fig. 228. — Same as Figure 227, Except on Upper Jaw.

cation. This cuspid papilla is missing, and the bicuspids have advanced
little beyond the form of the primitive bulb. The first molar is shown
with the full diameter of the crown represented by the pulpal mass, and
the tips of the cusps are already beginning to take on the calcific
action.

28o

ANATOMY.

A further illustration of the progress of the development of the
permanent teeth and their relation to the deciduous organs may be seen
in figure 227, the dissection in this instance being upon the lower jaw of
a one-month-old child. The membrane has been lifted from the bone
with the tooth-sacs attached to it. Immediately posterior to the sacs
containing the crowns of the temporary incisors and cuspids are those
,for their corresponding successors, the papillae of which have already
assumed the tubercular outline of the future cutting-edge. While the
permanent tooth-sacs are distinctively independent pouches, there ap-
pears, nevertheless, to be a well-established fibrous connection existing

Tooth-sacs of
Permanent Teeth

Tooth-sacs of
Deciduous Teeth

Periosteum of Hard Palate

FlG. 229.— Tooth-follicles fur Deciduous and Permanent Teeth, Three Months

after Birth.

between the outer layer of the two follicular walls. This fibrous union
is gradually broken as the permanent sacs recede and become incased
in their own vaults.

While the permanent tooth-sacs are generally referred to as "reced-
ing," it is a question if this term is fully justified. While the follicles
do not remain in close relation with their predecessors, the change in the
relative position of the two is principally brought about by the advance
in the deciduous sacs, this forward movement being accompanied by a
marked growth of the bone in the direction of the future alveolar ridge,
thus leaving the permanent tooth-sacs well buried in the substance of
the jaw.

Figure 229 represents the result of a dissection upon the superior
maxillae of a three-months-old child. The mucous membrane covering
the hard palate, together with the periosteum, has been dissected from
the bones and turned over for examination. The tooth-follicles for all

DEVELOPMENT OF THE PERMANENT TEETH. 281

the deciduous teeth, as well as those of the succedaneous permanent
organs, may be observed firmly attached to the fibrous tissue. The
permanent incisor sacs at this age are almost equal in size to those of
their predecessors, and the dentin papillae within possess a mesio-distal
diameter almost equal to those of the calcified temporary caps. The
sacs containing the germs for the permanent cuspids and biscuspids are
somewhat diminutive, but the enamel organs within are already molding
the contour of the future tooth-crowns upon the dentin papilla-.

By the beginning of the second month after birth calcification in the
crowns of all the deciduous teeth is about complete, and preparation for
growth of the roots is under way. While at this period the tooth-crowns
may be said to be almost completely calcified, this does not apply to the

Fig. 230.

interior of the crowns, the deposit of dentin internally being a continuous
process, resulting in a gradual reduction in the capacity of the pulp-
cavity. It is also quite probable that the enamel organ is somewhat active
up to the eruptive period, and, if this be true, the enamel covering of the
crown is not complete until this time. Whatever be the condition in the
crowns, the time for the formation of the roots has arrived, and it is princi-
pally through the activity of the tooth-pulp that they are generated.
We have seen that the contour of the tooth-crown was first molded upon
the dentin papilla; so it is with the tooth-root: by a gradual elongation of
the sac, accommodations are afforded the tooth-pulp for a corresponding
growth. As the pulp lengthens out toward the future apex of the root,
it is molded to the root-form, and calcification takes place by the genera-
tion of odontoblastic cells upon the periphery of this organic root-form.
While the process of root-formation in the single-rooted tooth maybe
readily comprehended, the bifurcation or trifurcation of the molar roots
presents a complication which calls for special reference. Figure 230
will assist in explaining this phenomenon. In the illustration three
deciduous molar crowns are shown, two of which are incased in their

282 ANATOMY.

tooth-sacs, the third being stripped of its membrane. The view is directly
upon the base of the tooth-sacs, immediately beneath which is the
base of the pulp. Up to this period the odontoblastic cells have been
generating about the occlusal surface and lateral walls of the crown only,
but now an accumulation of these cells is to be found upon the base of the
pulp, lining up in the position of the future root-walls. This structural
change is faintly outlined in the illustration. By this inward extension
of the odontoblastic cells from various points about the margins of the
pulp, and their union near the center of the mass, provision is made for
the calcification of the various roots, which process is considered separately
by an extension and molding of the pulp into two or more divisions.

Calcification of the Cementum. — While the dentin of the root is
derived from the tooth-pulp, the external covering of the root (the cemen-
tum) is generated from another source. In every respect cementum is
closely allied to bone, and we find its development provided for in a
similar manner. As stated in another part of this chapter, the tooth-
sac is made up of an outer and an inner layer, both of which are rich in
blood-vessels. These membranous walls continue to invest the roots
of the teeth during their upbuilding. The outer layer of the sac remains as
a permanent structure placed between the root and the alveolar walls,
forming the alveolodental membrane, while upon the surface of the inner
layer osteoblasts (cementoblasts) are developed, which are speedily
converted into bone or cementum. In this process the tooth-root may
be compared to one of the long bones of the body, and the development
of the cementum considered under the head of Subperiosteal Ossification.
The only variation to be observed between this and subperiosteal develop-
ment of bone is in the presence of a single Haversian canal (as the pulp-
cavity may be considered), and even this difference is sometimes over-
thrown by small canals running at right angles to the pulp. These small
canals are generally found near the apex of the root, at which point the
cementum is the thickest. Like the enamel cap of the tooth-crown, the
cementum is deposited upon the surface of the dentin of the root, thus
increasing its diameter.

Eruption of the Teeth. — -Up to this time no reference has been
made to that process by which the teeth burst forth from their bony incase-
ments, and, penetrating the mucous membrane, made their appearance
in the mouth. Attention has been called to the growth of the bone
about the tooth-follicles — first forming beneath them as an open gutter,
next surrounding their lateral walls and inclosing each follicle in a sepa-
rate compartment, and finally each tooth becoming more completely envel-

DEVELOPMENT OF THE PERMANENT TEETH. 283

oped by an arching-over of the mouth of the bony vault. This condition
in the maxillary bones is reached between the seventh and eighth month
after birth, and, almost simultaneously with the completed incasement
of the teeth by the bone, active resorption begins, that portion of the
bone which was last in forming being gradually removed. The cause
of the resorption of the bone may readily be attributed to the advance-
ment of the tooth, but the forces which are responsible for this latter
phenomenon do not appear to be clearly understood. In a general way,
the advancement of the crown may be said to result from the elonga-
tion of the root by the addition of dentin to its free extremity.
But, when it is taken into consideration that the cuspid teeth, both decid-
uous and permanent, have their roots fully or nearly calcified before
they begin to advance toward the surface, an exception to the generally
accepted theory is established. The eruptive process takes place first
in the anterior teeth,* and the bone overlying the labial surface is first
removed.

This loss of the bony structure is continued until fully one-half
of the labial surface is uncovered, and, as the crowns continue to advance
toward the surface, they assume a more prominent position in the arch,
and thus their cutting-edges become bared. The palatal or lingual face
of the crypt serves a double purpose, forming not only a covering to the
deciduous tooth, but also serving the permanent tooth-sac in the same
capacity. This part of the crypt remains unabsorbed, the tooth-crown
glides by its margin, and, after penetrating the mucous membrane, makes
its appearance in the mouth. Closely following the resorptive process
comes a rebuilding of the parts, until, finally, when the tooth is fully
erupted, it is firmly supported by the new bone filling in about the base of
the root. Accompanying the eruption of the anterior teeth and their
establishment in the arch is an increase in the depth of this portion of the
jaw, and, as the molar teeth advance and assume their position, there is
a corresponding increase in the depth of the jaw in this locality. At the
beginning of the eruptive period the roots of the deciduous teeth are but
partially calcified, but as the crowns advance the calcific action at the
extremity of the roots is continued, and, in the majority of instances, by
the time the crowns are fully erupted the roots are completely formed.
During the period of eruption the transitory nature of the alveolar por-
tion of the jaw-bone is made manifest, accommodating itself to the
growth of the teeth as well as to their change of position. The free
margins of the alveolar walls are taking on new structure, which ad-

*See Description of the Teeth in Detail.

284

ANATOMY.

varices with, and becomes adapted to, the base of the tooth-root. Coin-
cident with this the deeper portion of the alveolar process is formed
by a rapid filling-in about" the root as the tooth travels onward to assume
its final position in the jaw. The eruption of the teeth is usually by
pairs, with a slight intermission between each class. The central in
cisors first make their appearance, followed by the laterals, after which
the first molars are erupted. The cuspids usually follow the first
molars, and, finally, the second molars take their place in the arch.
While this brief description of the eruption of the teeth refers to the de-
ciduous organs only, the process in the permanent teeth is almost
identical with this. Further reference to the eruption of the perma-
nent teeth will be made in connection with the degeneracy of the
temporary set.

Fig. 231. — Hard Palate from a Nine-months-old Child, Actual Size

To return to the subject of tooth-development, attention is called to
figure 231, prepared from a dissection upon a nine-months-old child,
the illustration representing the hard palate, or roof of the mouth, at this
period. The four incisor teeth have made their appearance, the labial
surfaces of the crowns being fully exposed, while those facing the palate
are but slightly uncovered. The approach of the remaining deciduous
teeth is plainly indicated by the fullness of the alveolar borders, and the
margins of the crypts are now being removed by resorption.

If the mucous membrane should be removed, the crowns of the
advancing teeth would be brought to view after the removal of the walls
of the tooth-sacs, while the approaching cuspids and second molars yet
remain partly covered by an arching over of the walls of the crypts;

DEVELOPMENT OF THE PERMANENT TEETH.

285

the resorptive process has also begun in these parts. It would also be
observed that, while the walls of the crypts are molded to the outlines
of the tooth-crowns, there exists a well-defined interspace between the
two. During the growth of the tooth this interspace is filled by the walls
of the tooth-sacs, and even after the teeth have passed the saccular stage
of development, and assumed their positions in the mouth, there yet
remains between the roots and the alveolar walls a slight space which
is occupied by the alveolodental membrane.

The next dissection is one upon the superior maxilla? of a two-year-
old child (Fig. 232), representing the roof of the mouth of this subject.

Fig. 232. — Hard Palate from a Two-year-old Child, Actual Size.

In this specimen it will be noticed that all of the deciduous teeth are
erupted with the exception of the second molars. The lingual surfaces
of the incisors are fully uncovered, while in the cuspids the labial surfaces
are much more exposed than the lingual. In figure 233 the mucous mem-
brane and sufficient of the bone have been removed to expose the tooth-
sacs of the developing permanent teeth. The dissection furnishes no
additional information over that obtained from figure 232, excepting
that the primitive follicles for the permanent second molars make their
appearance at this time.

At this early period the jaw has not lengthened sufficiently to permit
of these follicles occupying their future position; consequently they are
found generating immediately over the tooth-sacs of the first permanent
molars. As the first molars advance and the jaw lengthens backward,
these follicles will be carried to the distal by the extension of the mucous

286

ANATOMY.

membrane, to which they are firmly adherent. If these follicles were to
be dissected at this time, the papillae would be without definite form,
showing that the early function of the enamel organ has not yet begun.
The tooth-sacs containing the permanent lateral incisors are found
immediately beneath the palatal plates, and frequently during their
earlier life they are not even protected by the bone, being in immediate
contact with the mucous membrane. On account of the imperfect
protection frequently afforded these sacs, the germs are sometimes
injured and the teeth fail to make their appearance.

Sac for Perma-
nent Second
Molar

Fig. 233. — Roof of the Mouth of a Two-year-old Child.

In figure 234 the walls of the sacs shown in figure 233 have been opened,
and the relations existing between the first and second dentition at the end
of the second year become apparent. The crowns of the permanent in-
cisors are deeply set in the substance of the jaw, while the partially calci-
fied crowns of the permanent laterals are in close proximity to the palatal
surface. The partially formed crowns of the permanent cuspids are still
more deeply seated in the substance of the jaw than those of the central
incisors, and are not visible in the illustration. In this connection it will
be well to again refer to the gubernaculum, and to its function — that of
directing the tooth to its proper position in the arch. By reference to
the illustration the crowns of the permanent teeth will be observed head-
ing in various directions, and, while in this instance there appears to be a
general tendency for them to advance and assume their proper positions
in the arch, in many cases they will be found directed at right angles to
the point at which they should emerge from the bone. The gubernaculum,

DEVELOPMENT OF THE PERMANENT TEETH.

287

which appears to be nothing other than an elongation of the follicular
walls, not only directs the tooth by the tension of its fibers, but the fora-
men which its presence creates stimulates the resorptive action over the

Fig. 234. — Development of the Teeth about the Second Year.

tract to be traveled by the tooth. Figure 235 was prepared for the pur-
pose of better showing the gubernacula, and the manner of connecting
the tooth-sacs with the oral mucous membrane. This condition is

Guber-
naculum

Tooth-sac for First Bicuspid
Fig. 235. — Dissection of Lingual Face of Lower Jaw, Child Nine Months Old.

well shown in the anterior teeth, the tooth-crowns having receded well
toward the body of the jaw. The follicle for the first bicuspid may be
observed attached to the lingual face of the deciduous molar sac, and

288

ANATOMY.

the leading cord is not yet an adjunct to the developmental process, but
this structure will make its appearance as the follicle recedes from the
surface.

In figure 236 the deciduous molars have been removed from the jaw

and the relations existing between
these teeth and the developing
bicuspids is shown. The tooth-
follicles for the succedaneous teeth
are found immediately beneath the
gingival margin, and apparently
attached to the deep layer of the
mucous membrane. This relation-
ship between the permanent and
temporary organs is present about
the eruptive period, but as the
deciduous tooth advances and the permanent tooth-sac recedes, the
two organs become more widely separated, and the permanent follicle
is connected to the surface only by the elongated follicular fibers which
form the gubernaculum.

Decalcification of the Deciduous Teeth. — By the close of the
second year the twenty deciduous teeth have taken their place in the

Fig. 236. — Deciduous Molars with
Tooth-sacs for Bi< uspids Attached to the
Gingiva] Tissui .

Deciduous Molars Deciduous Incisors

Permanent
Incisors

Permanent
Cuspid

Fig. 237. — Same as Figure 235, with Tooth-sacs Opened Showing Developing

Teeth in the Jaw .

dental arch, their roots have become fully calcified, and the apical fora-
mina established; it is only for a short period, however, that they remain
thus perfect, the process of decalcification beginning about the fourth
year. This resorptive action begins at the apical extremities of the roots

DEVELOPMENT OF THE PERMANENT TEETH.

289

and gradually progresses in the direction of the crowns. Commencing
about the fourth year with the central incisor, decalcification takes place
in the teeth in the order of their eruption, the lateral incisor following the
central, the first molar following the lateral, etc. By reference to figure 238
an approximate idea of the progress of decalcification may be obtained,
and it will be observed that about three years elapse from the beginning
of this rather obscure process to its completion, and the final casting-off

Fig. 238. — Development of the Teeth about the Sixth Year.

or shedding of the tooth-crowns. In reference to the causation of this
dissolution of the deciduous teeth, but little appears to be known. It
has been said to result from the presence and pressure of the advancing
permanent teeth, but there is no question but that it. occurs absolutely
independent of these organs, decalcification frequently taking place when
from some obscure reason, one or more of the successional teeth are
absent. During the entire period of root decalcification, the pulp of the
19

2 go

ANATOMY.

tooth, which is also involved in the destruction, retains its vitality, but
with the loss of vitality in the pulp resorption of the root ceases; so that
the gradual removal of the root-substance must be considered as a purely
physiologic action.

Figure 238 shows a dissection upon the jaws of a six-year-old child,
by a careful study of which, a fair knowledge of the extent of resorption
in the deciduous teeth at this period may be obtained.

Advance of the Permanent Teeth. — By referring to figure 238, the
relations existing between the deciduous and the permanent teeth at

FlG. 239. — The Completed Dentition.

about the sixth year may be noted. While the crowns of the deciduous
teeth remain in position, a part of the space formerly occupied by their
roots is taken up by the advancing crowns of the permanent teeth, the
latter being calcified but little beyond their cervical lines. Between the
seventh and eighth years the crowns of the deciduous incisors are cast
off, and gradually the crowns of the permanent incisors force their way
through the gum, the arch by this time having sufficiently increased in
size to accommodate the additional width possessed by them. Previous
to this time, or about the sixth year, by a backward extension of the jaws,
the permanent first molars have erupted, assuming a position in the arch
immediately posterior to the deciduous second molars. Between the

DEVELOPMENT OF THE PERMANENT TEETH. 29 1

tenth and eleventh years the crowns of the deciduous molars are lost, and
the bicuspids advance to take their places. Usually by the twelfth year
there has been sufficient increase in the length of the jaws to permit of
an additional tooth, and the permanent second molar gradually takes its
position immediately posterior to the first. Between the twelfth and
thirteenth year the deciduous cuspids are lost by decalcification of
their roots, and they are succeeded by the permanent cuspids. We
therefore find, by the fifteenth year, fourteen fully developed teeth occupy-
ing the dental arch of each jaw, the full number, thirty-two, or sixteen
in each jaw, not being present until the eruption of the third molar,
which, like the other teeth of this class, is compelled to await accommoda-
tions by a further increase in the length of the maxillary bones. This
tooth usually takes its place between the eighteenth and twenty-first
years, and thus completes the dentition (Fig. 239).

PART II.-HISTOLOGY AND HISTOGENESIS.

CHAPTER I.

General Cytology; General Embryology; and Histogenesis.

The foregoing pages have been devoted to the description of the face
and oral cavity, so far as they can be studied with the naked eye, either
by close inspection, superficially as it were, or after their component parts
have been made manifest by dissection. That part constitutes what is
commonly known as gross or macroscopic anatomy.

Similar to all other regions of the body, however, the functions of
the individual parts composing the face and the oral cavity, in health,
and the pathological processes which take place in them when they are
diseased, are dependent on the normal or pathological condition of the
minute structures or tissues, of which these parts are composed. These
minute structures cannot be seen with the naked eye, they must be exam-
ined with the aid of a microscope, and this part of study is therefore
known as microscopical anatomy or histology — the science of tissues.

The minute structures of the face and oral cavity, generally speaking,
in no way differ from the structures of other parts of the body, but here,
more than in any other region of the body, the anatomy as well as the
histology of the parts cannot be well understood, unless their gradual for-
mation or development — embryology — is studied. To enable the student
of dentistry to understand more intelligently the details of the parts in
which he is most interested, it is therefore essential to have a more or less
thorough knowledge of the broad principles underlying, not only the
mature structural arrangements of the human body, but also the develop-
ment of the organs — organogenesis — as well as the development of the
tissues of which these organs are composed — histogenesis.

In submitting parts of different organs of the body to a more close
examination with the microscope, we find revealed a great variety of
more or less complex textural arrangements, each characteristic of the
part examined, and adapted to the function which the given part has to
fulfill. Some tissues we find consisting of either very thin or more or

2 93

294 HISTOLOGY.

less coarse fibers running parallel to each other in a compact fashion and
forming firm or elastic bands; others we find showing the same kind
of fibers loosely arranged and interlacing with each other, thus forming
more or less typical networks. In some instances we find similarly
formed elements, so-called cells, joined together in a mosaic-like fashion
and forming either coverings of surfaces or various tubes or sacs. The
majority of organs consist of fibers and cells, modified and combined
together in either a simple or more or less complex manner. We also
observe that the substances filling the spaces between the fibers and
cells, the so-called ground substances, vary in their consistency; it maybe
a fluid or a semifluid, it may be of a more or less firm nature, or it may
be very hard. Notwithstanding this great variety, however, all the
tissues of which the animal body is composed can be grouped into five
distinct classes, each one of which has its very well defined character-
istics of structure, which makes it specially adapted to the various func-
tions which it has to fulfill in the animal economy. These classes com-
prise the elementary tissues, which by their various modifications and
combinations form all the organs of the body. They are the following:
i. Epithelial tissue. 2. Connective tissue. 3. Muscular tissue. 4. Nerv-
ous tissue. 5. Blood and lymph.

Before we attempt to describe the individual elementary tissues,
it is necessary to become familiar with some features which are common
to all of them. This can be best accomplished by giving a brief history
of the beginning and gradual development of our knowledge on the subject.

The principal features in the minute structure of animals are the
same as of plants, and it may be justly stated that animal histology
took its origin from the histology of plants. In 1667 Robert Hooke
published the results of his microscopical investigations of a piece of cork.
He found that it consisted of a number of small boxes or cells, and thus
the name cell was introduced into histologic nomenclature. In 1838
the botanist Schleiden published the results of his elaborate studies of
plant structures, and established the fact that, no matter how widely
individual plants and parts of plants differ from one another in their
general appearance, in their minute structure they reveal themselves as
constituting aggregations of cells, or modifications of such. He however
laid particular stress upon the fact that each cell contains a substance,
semifluid in nature, which constitutes the essential, life-carrying part
of the cell, which by later investigators was named protoplasm. Further-
more, he found that within the protoplasm there can always be distin-
guished a kernel-like body, which was named nucleus.

GENERAL CYTOLOGY; EMBRYOLOGY; AND HISTOGENESIS.

2 95

Inspired by personal contact with Schleiden, the anatomist Schwann
undertook extensive examinations of various tissues of animal bodies.
In 1839 he published the results of his investigations and established the
fact, that no matter how widely various animals and organs of animals
differ in regard to their general appearance, in their minute structure
they always present aggregations of cells, or modifications of such.

This conformity in the results of the investigations of Schleiden and
Schwann has revealed the most interesting as well as the most important
fact, that just as all chemical compounds have as their ultimate units

Fee 240. — Cells from bulb of a fresh onion forming a membrane M, cell membrane; N, nu-
cleus seen from the surface; Nn, nucleolus; V, vacuoles, X 240. (After Stirling.)

the atoms or the molecules, so all living tissues have as their ultimate
units the cells. This forms the foundation for all considerations of
plant and animal histology up to the present time, notwithstanding
the recent advancement in our knowledge of the structure and life of the
cell itself.

According to the views of Schleiden, Schwann and their contempo-
raries, a cell consists of: (i) a cell- membrane; (2) a substance contained
within — a protoplasm; and (3) a small kernel-like body — a nucleus.

Continuous extensive studies of the structure of cells in various
animal tissues have, however, very soon revealed the fact that while in
all cells there can always be demonstrated a protoplasm and a nucleus,
the presence of a cell-membrane is an exceedingly rare occurrence, and
that this part therefore cannot be considered as an essential one in the
structure of a cell. It was also found that there may be a number of

296 HISTOLOGY.

other structural elements present in one or the other kind of cells, but by
their absence the individuality of the cell is by no means lost. The
establishment of these facts naturally led to a change in the conception of
the cell-structure, and in 1861 a distinguished investigator, Max Sclmltze,
narrowed down the definition of a cell to a mass of protoplasm con-
taining a nucleus, and this definition is generally accepted as the most
appropriate one up to the present time.

Another very important point pertaining to the conception of the
cell has attracted the attention of all well-known investigators for a long
time, namely, the origin of cells in general, and the formation of new cells
in the different tissues for substituting old ones when they are worn out
or lost through injury or disease. It was at one time believed that cells
may originate spontaneously by transformation from organic or inorganic
chemical compounds. By persistent investigations it was however
gradually determined that the so-called spontaneous generation is much
less frequent than had at first been supposed, and at present the belief
in the formation of cells in that way is scarcely held by anybody. It is
definitely established that all varieties of animal cells, no matter to which
class of tissues they may belong, always originate from preexisting cells
of the same kind. Furthermore, it was ascertained that while the
different tissues of the animal body can be easily distinguished from one
another in their mature state, there are great similarities between them
in the early periods of the development of the animal. These similari-
ties become the more pronounced the further back we trace them to
their starting-point, and find that all tissues collectively take their origin
from the same source, namely, from the ovum or egg, which, according
to our knowledge at present time, is also nothing else but a single cell.

The polymorphism of cells, or the great variety of cell-forms, which
was observed in the animal body, has naturally suggested the assumption
that the structure of cells with their essential and accessory parts
is by no means a simple one, and further investigations have proved this
to be correct. The study of cells and tissues has been greatly facilitated
by the discovery of various substances, animal as well as vegetable, and
particularly chemical compounds, which, when brought in contact with
the tissues, show a distinct chemical affinity to the different parts of it
and possess the ability of staining them in a distinct characteristic manner.
Some of these substances stain the protoplasm of the cells and are
therefore called protoplasmic stains, others stain the nucleus and are there-
fore called nuclear stains. By application of these staining materials it
was made possible to reveal the details in the structure of the various

GENERAL CYTOLOGY; EMBRYOLOGY; AND HISTOGENESIS.

297

cells, and in the following we will give a description of the minute struc-
ture of the cells, as it is generally accepted at present time, and may be
observed in one or the other kind of physiologically differentiated cells
during the various stages in their life-history.

According to the statement made above, the essential parts of a
cell are the protoplasm and the nucleus. The term protoplasm is how-
ever used in modern histology to designate the substance of the whole
cell, while the substance of the cell-body surrounding the nucleus is
termed cytoplasm; the substance of the nucleus is sometimes termed
nucleoplasm, but most frequently simply nucleus. We will also use these
terms in our description.

Nuclear membrane

Achromatic
substances
of the nu-
cleus

Linin

Nuclear sap

Centrosome

"-Inclusions

Cuticular stratum —

Filar-mass

Interfilar-mass .

Nuclein
cords

Netknots

- -Nucleolus

" Microsomes

Chromatic
substances
of the nu-
cleus

- ■ Cell membrane (pellicula)

Fig. 241. — Scheme of a Cell. Microsomes and filar-mass only partly sketched. (Stokr.)

Cytoplasm. — The structure of the cytoplasm has been described
differently by different investigators, and several theories have been
advanced in that respect. Some have described it to be of an homo-
geneous nature; others found it to be reticular; again, it was claimed to
be granular; by some it is supposed to be foam-like. Thanks to experi-
mental research-work carried out within recent years, it is positively
known, at present, that the most important of all vital manifestations of
the cells — metabolism — takes place in the cytoplasm, and this being the
case the various stages of the process naturally manifest themselves in the
structure of the cytoplasm. In this, and in the diversity of the methods
used in preparing tissues for examination, must be sought the cause for

298 HISTOLOGY.

the difference in the results obtained. There is a general agreement
among histologists, however, that even when fresh, unstained tissues are
examined there can be recognized in the cytoplasm two different parts,
one forming a reticulum or network and called spongioplasm; the other,
a substance more homogeneous in character, which fills out the spaces
of the network and is called hyaloplasm. The reticulum of the spongio-
plasm may at times show a bead-like structure, thus presenting a granular
appearance. The hyaloplasm may also be not entirely homogeneous,
but contain very fine granules which are known as microsomes. There
may be also found in the cytoplasm various foreign inclosures, such as
particles of pigment, droplets of oil, some apparently empty spaces known
as vacuoles, etc. All such inclosures are collectively known as meta-,
para- or dculoplasm. There is also very often seen a condensation of the
peripheral part of the cytoplasm, constituting what is called an ecto- or
exoplasm, which without any sharp lines of demarcation passes into the
more central part, called endoplasm. A cell-wall or -membrane must be
considered as a highly specialized part of the exoplasm.

Nucleus, — In the great majority of instances, the nucleus is a spheri-
cal or oval body, situated usually either in the center of the cell or nearer
to one pole of it than to the other, and varies in size generally in propor-
tion with the size of the cell. In general its structure may be said to be
somewhat similar to that of the cytoplasm, as it also consists of a reticu-
lum or network, and a substance, more fluid in nature, filling the meshes
of it. The network of the nucleus is however more complicated in its
structure. By means of staining the specimens with so-called nuclear
stains it has been revealed that the nuclear network consists of two
different substances, first, of an exceedingly delicate, non-stainable reticu-
lum, apparently very similar to the one of the cytoplasm, and called
linin; secondly, of a substance which has the form of threads, or of a
network, or of a mass of granules, and has the power of absorbing very
actively certain dyes. This latter substance has therefore received the
name chromatin substance in contradistinction to all other substances
of the nucleus, which are not affected by the dyes, and therefore known
as achromatin substance.

The nucleus is separated from the cytoplasm by a distinct line of
demarcation, which is known as nuclear membrane. This may however
disappear at times, thus making possible an interchange of the substance
of the nucleus with that of the cytoplasm. It generally takes place
during the process of the so-called cell-division, of which we will speak
later.

VITAL MANIFESTATIONS OF CELLS. 299

Nucleolus. — This is a small body found in the center of the nucleus
in the great majority of cells. In some instances, as for example in nerve
cells, it becomes very conspicuous. The significance of it in the cell-
life is however not as yet well established.

Centrosome. — This is a minute body observed in cells in close relation
to the nucleus and generally situated just outside of the nuclear membrane.
It consists of a more homogeneous substance and contains in its center a
small dot, the centriole. The centrosome can not always be seen, but it
becomes very conspicuous during a certain period of activity of the cell,
namely, during cell-multiplication, when it plays a very significant role.

Other constituents of cells have been mentioned already.

VITAL MANIFESTATIONS OF CELLS.

In animals consisting of but one cell, the so-called unicellular organ-
ism, all life functions are naturally exerted by this one cell. The results
of microscopic and experimental investigations of the last half century
have however led to the conviction that the life activities of all the familiar
higher animals, in health as well as in disease, are dependent on the
activities of the cells of which they consist. It is for that reason that,
while the study of the details of this subject belong to the domain of
physiology, some of the more important points of it are also generally
considered in connection with the description of cell-structure. The
most conspicuous vital characteristics of cells are the following:

Metabolism. — Of all the vital activities of cells, this is the most essen-
tial one. It represents the sum total of all those physico-chemical proc-
esses taking place in cells which have as their ultimate result the main-
tainance of the life and individuality of the cells. It therefore embraces
the process of taking into the body material suitable for its nutrition;
the process of converting some of it into a part of its own substance to
replace that which became worn out; transformation of another part
into various kinds of energy; finally, the elimination of substances, not
suitable for further use, as waste-products.

Growth. — Newly-formed cells are generally not of the same size as
mature cells, therefore a part of the substances assimilated by young cells
is used up by them for the increase of their size. This goes hand in hand
with the shaping of the cell until it becomes identical with the one from
which it originated.

Irritability. — This is a faculty of living cells to respond to various
influences: mechanical, chemical, thermical, etc. It generally results in

300 HISTOLOGY.

those peculiar manifestations of the cells to which they are specially
adapted, for example, secretion, motion, etc.

Conductivity. — This is the ability of cells to convey and transmit
impulses from one part of the cell to another and from one cell to another.

Motion. — This is the property of cells to change their relative posi-
tion to the surrounding media. It occurs in three different forms:

(i) Amoeboid Motion. — Similarly to the amoeba, cells may send out thin
or thick projections from their body, called pseudo podia, and if such pro-
jections become attached to some foreign object and the rest of the body
is drawn after it, there results a creeping motion. Cells endowed with
such motion are known as wandering cells. In some instances such cells
may, by means of their processes, surround foreign bodies and either
incorporate them by assimilation into their own body-substance, or
deposit them by expulsion into various other localities. Such wander-
ing cells are known as phagocytes.

(2) Ciliary Motion. — Some cells, usually those columnar in shape,
have one of their free surfaces beset with a number of delicate, transparent,
hair-like processes called cilia, which are constantly lashing forward
and backward (reminding of the blades of grass in a field, when acted
upon by a strong wind) producing a strong current, thus sweeping along
various small substances.

(3) Contractile Motion. — This is generally observed in fiber-shaped
cells, such as are found in muscles. It consists of a shortening of the fibers
in the direction of its long axis. The result is an approximation of the two
ends of the fiber and consequently an approximation of the parts to
which the two ends are attached.

Reproduction. —While all the just mentioned cell-properties either
serve to maintain their own individuality, or are the manifestations of
their function in the animal economy, reproduction is the property of
cells to form new cells similar to their own, a kind of rejuvenation, as it
were. The object of this is either to substitute new cells for the ones
which have become worn out and cast off, or, by adding new ones to their
own ranks, increase their functional ability.

The studies of the structural changes which take place in cells
during cell-reproduction, have been pursued with great zeal and enthu-
siasm during the last four or five decades, because it was through these
studies that the true and complex structure of cells was revealed. We
will therefore consider this phenomenon in its details at this point.

The cell-structure, as described on page 298, is considered as the
one which presents the cell in its so-called resting state, and the changes

VITAL MANIFESTATIONS OF CELLS.

3OI

which have been observed in it during the process of reproduction are
the following:

In the first place, the centrosome, whether it has previously been visible
or not, becomes very distinctly defined, and divides in two, which grad-
ually separate from one another, and ultimately pass to opposite poles

,nu.

* . -SD. .' %'<;::VC *
/--chr. ', ^fj^iPir

* -:p -

nu.

..sp.

Fig. 242. — Diagram of indirect or mitotic cell-division; A, cell with nucleus in resting stage;
B, cell with nucleus in skein stage and with centrosomes separating; C, formation of chromo-
somes; D, longitudinal splitting of the chromosomes; E, separation of the chromosomes;
F, diaster stage ; G, H, formation of the two daughter cells; c, centrosome; cl, chromatin thread;
chr., chromosomes; nu, nucleus; n, nucleolus; sp., nuclear spindle; w, cell wall. (After
Calloway.)

of the cell; they remain connected with each other however by means
of finely drawn-out non-stainable fibers, known as the achromatic spindles.
In the meantime the nuclear membrane disappears and the chroma-
tin substance of the nucleus, whatever be its form during the resting
stage, assumes the form of a continuous, densely coiled-up thread, which

302 HISTOLOGY.

is known, as the close skein. This coil gradually becomes in a measure
entangled, and is then known as the loose skein.

The next step is one of the most significant ones in the whole process.
The whole chromatin thread becomes broken up into a number of seg-
ments known as chromosomes, and it is a very important fact that, while
the number of chromosomes varies in cells of animals of different species,
there is always an equal number of them in cells of animals of the same
species. The chromosomes become arranged in the equatorial plane
of the spindle in a somewhat star-like fashion, and this is then spoken of
as the monaster stage. Next, a splitting of each individual chromosome
in its longitudinal direction takes place, and in this way the number of
chromosomes becomes doubled, each one consisting now of a pair, or
presenting a twin-chromosome, as it were. The two chromosomes of
each pair gradually separate from one another and one of each pair ulti-
mately passes to opposite poles of the spindle, near the centrosome. Here
they also assume a star-like arrangement, and we have there what is
known as the diaster -stage. The chromosomes are generally regarded at
the present time as the bearers of heredity.

The changes, which take place in each one of the stars, from now on
present simply the reverse of those described above for the formation of
the monaster, i.e., the individual chromosomes become united with one
another, thus forming two skeins. At first the skeins are loose, then a
condensation takes place and two close skeins are formed. Finally,
around each one of them a membrane is formed, and there appear two
nuclei similar to the one from which they originated.

While this final shaping of the nuclei is going on, the cytoplasm of
the cell becomes constricted half-way between the two nuclei, and when
this is completed a division has thus taken place, and we have two cells
instead of the original one.

The various investigators have laid paticular stress on one or the
other stage during the process of cell-multiplication. Accordingly, various
names have been given to the same process, they all have their justifica-
tion and are in use as synonyms. Taking into consideration the ultimate
result of the process, it is called cell-division. In view of the fact that
the changes are most conspicuously going on in the nucleus, it has been
spoken of as karyokynesis. Because the chromatin substance of the
nucleus assumes the form of a thread, the process is called mitosis. Finally,
as this form of cell-division is accomplished in a rather complicated
round-about way, it is also known as indirect cell-division.

While the process of cell-division just described is the most frequently

ORGANS AND TISSUES.

3°3

met with, there can be observed a more simple one, which leads to the
same end. Without any preliminary rearrangement in its structure, the
nucleus of the cell becomes elongated, then it assumes a dumb-bell shape,
finally the connecting neck becomes broken across and thus two so-called
daughter nuclei are formed which gradually separate from one another;
in the meantime a constriction of the cytoplasm takes place, and as a
result there appear two cells instead of one. For the reason that in
this form of cell-division the nuclear substance does not assume any
thread-like arrangement, and the end is accomplished in a rather simple

Fig. 243. — Direct cell division (Amoeba). — A, active specimen with pseudopodia; B, becom-
ing spherical preliminary to division; C, beginning of elongation and constriction; D, later
stage; E, daughter cells forming pseudopodia; ec, exoplasm; en, endoplasm; /, food particle;
n, nucleus; ps, pseudopodium; v, vacuole. (After Galloway.)

manner, this process is called amitosis or direct cell-division. This
process has been observed in the white corpuscles of the blood, occasion-
ally in the liver, and not infrequently in pathologic conditions; it is assumed
that this mostly occurs in cells having a lowered vitality, there is
however much to be learned yet in regard to this question.

ORGANS AND TISSUES.

All through the animal kingdom, with the exception of the unicellular
organisms, we observe, among the various elements of which they consist,
the manifestation of a phenomenon commonly known as division of
labor. We find groups of cells united together in various fashions to
perform a certain function, and such aggregations of cells, specialized to
fulfill well-defined duties in the economy of the organism, are called organs.
Only among the lower animals, however, do we find that the organs consist

304 HISTOLOGY.

of only one kind of cell. Higher in the scale of animal organization, we
find the organs exhibiting more and more complexity in their make-up.
They consist of groups of various kinds of cells, and the individual groups
present very little similarity to each other, sometimes becoming modified
to such an extent that it is very difficult to recognize the cellular nature
of them. While the name organ conveys the idea of a physiological unit,
the texture of it is known as tissue. If the tissue presents an aggregation
of elements, similar in character, we speak of simple tissues; and where
we have an aggregation of elements of various kinds, we speak of complex
tissues.

To gain a proper understanding of the characteristic appearance of
the various simple tissues, their various modifications, combinations and
transformations in forming that multitude of structures, which is ob-
served in a mature organism, it is .necessary to trace them to their first
beginning, or histogenesis.

We have stated elsewhere that an animal body takes its origin from
the egg, ovum, which is nothing else but a single cell. While ova of
various types of animals differ in size, they all have relatively the same
structure and are all specialized for the same purpose — to produce a new
individual. The changes which an ovum undergoes in course of its
development into a new individual are in all cases principally the same,
and therefore the knowledge gained from the study of one form serves us,
in a general way, to understand all others. The study of these consecu-
tive changes constitutes the subject of General Embryology and
Histogenesis.

General Embryology and Histogenesis.

In all animals with sexual mode of reproduction, the cell from
which the whole organism ultimately develops is itself a product of
the union of two highly differentiated and specialized cells; one, supplied
by the male individual and formed in the testicle — the spermatozoon; the
other, supplied by the female individual and formed in the ovary — the
ovum. These two so-called elements of reproduction are derived by trans-
formation of special cells in the body, the so-called germinal cells, which
are set aside, as it were, for the purpose to be eliminated from the body
at certain periods and used as a foundation for the propagation of the
species. The process of entrance of the spermatozoon into the ovum,
and the ultimate union and fusion of the two elements is known as
the fertilization. Before this fertilization can take place, however, the

GENERAL EMBRYOLOGY AND HISTOGENESIS. 305

ovum has to undergo a preliminary process called maturation, which
consists in the elimination of half of the quantity of its nuclear substance
in form of two small bodies, expulsed at one of the poles of the ovum,
and therefore called polar bodies. 1 The fertilized ovum is that cell from
which a new being gradually develops; and while the mode of develop-
ment varies somewhat in its details in different species of animals, the
general principles of the process always remain the same. In the follow-
ing we endeavor to give a short general account of the essential points

Fig. 244. — Four stages in the maturation and fertilization of the ovum (partly diagram-
matic). A, formation of the polar bodies and entrance of the spermatozoon; B, the male and
female pronuclei; C, nuclei coming together; D, pronuclei uniting to form segmentation
nucleus; e.n., egg nucleus; p. 6., polar bodies; s, spermatozoon; s. c, sperm centrosome;
s.n., sperm nucleus; 5. e., segmentation nucleus produced by the union. (After Galloway.)

in the development of the ovum, but for the details of it we must refer
to the various text-books on embryology.

After a short period of rest, the fertilized ovum begins to undergo
the so-called process cleavage or segmentation. By means of karyo-
kinesis it divides into two halves, producing two cells. Each of these cells
in turn again divides, giving rise to jour cells, and this is succeeded
by another division, forming eight cells, and by repeated division of this
kind there arises a solid mass of smaller cells called morula or mulberry
mass, from its resemblance to a berry. As the cells increase in number,
the mass also increases in size by the absorption of nutriment, and a

1 By expulsion of the polar bodies the number of chromosomes in the ovum is reduced
to one-half of it; the fusion with the spermatozoon, which is also supplied only with half the
number of chromosomes, the normal quantity is restored and the fertilized ovum thus con-
tains heredity bearing substance from both its progenitors.
20

3° 6

HISTOLOGY.

gradual arrangement of the cells in a definite fashion takes place, varying
however with the character of the ovum. They may arrange themselves

H I

Fig. 245. — Diagrams showing the development of the germ layers. (After van Beneden
and St oh r.) A, Two-celled stage; B, four-celled stage; C, morula stage; D to /.transverse
section of various stages of the blastoderm.

in the form of a layer of cells spherically surrounding the so-called seg-
mentation cavity; or they may be spherically arranged around a mass of
yolk; or they may form a disk-like arrangement, floating as it were on

GENERAL EMBRYOLOGY AND HISTOGENESIS. 307

a spherical mass of yolk. In any case we observe the cells to be cylin-
drical in shape and, lying side by side in regular fashion, form a some-
what skin-like arrangement. This is therefore called the blastoderm,
which means a germinal skin, and the individual cells are named blasto-
meres. The continuous multiplication of the cells of the blastoderm
results in either an infolding (invagination) or a splitting off (delami-
nation) of some of the cells, which by their own gradual multiplication
form a distinct continuous layer of cells beneath the first one. It is
then generally spoken of as a two-layered blastoderm, of which the
outer layer is called ectoderm or epiblast, and the inner layer entoderm
.or hypoblast. These two layers are called the primary germinal
layers, and the cells of which they consist show great similarities in
many respects. Very soon, however, some of the cells, on the surfaces
of the two primary layers which face each other, detach themselves
therefrom by the process of splitting off or delamination and lodge in
the gradually widening space between the layers. As the number of
these cells increases also through their own multiplication, it very soon
becomes possible to distinctly recognize a third layer of cells, which, on
account of its being situated between the other two, has received the
name of middle layer or mesoderm or mesoblast. With the formation
of the mesoderm, the blastoderm is said to consist of three germinal layers,
and with this the foundation for the development of the various tissues
and organs is finally established. The resemblance of the cells consti-
tuting the different layers gradually becomes more and more lost, and
differentiation then begins to take place. Through all the consecutive
changes, the derivatives of the ectoderm and entoderm retain the charac-
teristic tendency to be arranged in groups of cells lying closely side by
side in a simple or stratified fashion. The derivatives of the mesoderm
are, on the contrary, characterized by a looser arrangement of their cells,
some of which remain connected with one another by means of proto-
plasmic prolongations or processes, while others retain the faculty of
wandering away and intermingling with the derivatives of the other two
layers. Each of the three layers gives rise to well-defined structures
adapted to distinct physiological functions, but none of them can in this
respect be substituted by another, without producing abnormal conditions
in the organism.

CHAPTER II.

Elementary Tissues: Epithelial Tissue, Connective Tissue,
Muscular Tissue, Nervous Tissue, Blood and Lymph.

The differentiation of the three layers of the blastoderm into various
tissues goes on parallel with the development of the body in general, and
with the growing complexity of organization along the scale of the animal

cnt- -

u B

Fig. 246. — A, Diagram of a longitudinal section through the body of a Hydra; it presents
the structure of an animal, in which the walls consist of only two layers of cells specialized to
perform all the function. B, a small portion of the wall more highly magnified. (After
Galloway.)

kingdom, the number of varieties of tissues as well as their complexity
also increases. On page 294 we have already indicated, however, that
we can resolve all the various tissues into five well-characterized typical
groups, which are known as elementary tissues. These are: (1) epithelial

308

EPITHELIAL TISSUE. 309

tissue, (2) connective tissue, (3) muscular tissue, (4) nervous tissue,
(5) blood and lymph.

Among the lower animals there are some which resemble to a
considerable extent the early embryonic stages of higher animals.
Their walls consist of two layers of cells, which resemble one another
very much, and are transformations respectively from the ectoderm and
entoderm. They are endowed with the ability to perform all functions
which characterize living bodies and represent that typical group of cells
known as epithelial tissue. Accordingly, in the higher animals, also, the
principal functionating parts of the various organs, including the nervous
system, are by various transformations derived from the ectoderm or
entoderm, and are either epithelial tissue proper or highly specialized
modifications of it.

The mesoderm begins to develop when the other two layers are
already well differentiated, and, accordingly, the tissues which take
their origin from that layer are found very sparingly, or not at all, in
lower animals, but become more and more conspicuous as we ascend in
the complexity of animal organization. It gives rise mainly to tissues
which, while not constituting the principal parts in the texture of various
organs, are nevertheless of very great importance to them and form indis-
pensable auxiliary parts of them. These are the connective tissues,
muscular tissue and blood and lymph. Some epithelial formations take
their origin in the mesoderm also, but their number is very limited.

We will now consider the individual tissues in detail.

I. EPITHELIAL TISSUE.

Epithelial tissue is found as coverings of all surfaces of the body,
those directly exposed to the air as well as those which form variously-
shaped cavities and communicate with the air indirectly through narrow
or wide openings. The cells composing the epithelial tissue are known as
epithelial cells. These are definite in outline, show very clearly a cytoplasm
and a nucleus, and lie side by side in a regular fashion. There is just
enough substance — so-called intercellular substance — between the individual
cells to hold the cells together. Three principal forms of cells are gener-
ally met with: (1) the flattened or squamous, (2) the cylindrical or columnar,
and (3) the many-sided or polyhedral. If a single layer of cells only is
present, it is known as simple epithelium; if there are two or more super-
posed layers of cells present, it is spoken of as stratified epithelium. In
simple squamous epithelium the cells are flattened or scaly and the

3IO HISTOLOGY.

nuclei are round and also flattened. In the stratified squamous variety,
which is the one most frequently met with, only the superficial layers are
squamous, while the deeper ones are more irregular, and may gradually
become columnar.

In columnar epithelium there is quite a variety in the outline of the
cells. In simple columnar epithelium the cells may be either long —
high columnar — or of a medium size — cuboidal — or very short- — low colum-
nar. They may be beset on their free surfaces with numerous minute
hair-like processes, which are constantly vibrating during life and are
known then as ciliated epithelium. In the stratified variety also the
superficial layer only may be typically columnar, while the others may be
of a different shape. Among a continuous superficial layer of columnar
epithelium, there may be found scattered here and there cells which have
somewhat the shape of a conical cup and contain in their cytoplasm mucus
in various states of formation. From time to time a contraction of their
cell-body takes place, and their content, which is of a mucous character,
is poured out upon the surface. Owing to their shape, these cells are
called goblet cells.

The polyhedral epithelium may be found in various localities in the
body. If the body of the cells contains some pigment granules, it is
known as pigmented epithelium. When polyhedral or columnar epithe-
lium is forming the constituting part of the so-called glandular organs, it
is known as glandular epithelium. A very conspicuous variety of epithe-
lial cells of various shapes is represented in the so-called neuro-e pilhelium ,
which is specialized for the creation and perception of the special senses.

Epithelial tissue never contains blood-vessels; their nutrition takes
place by the absorption of nutritive juices through the clefts between the
cells, or the cement-substance. It is obvious that when a stratified
epithelium consists of a large number of layers, the superficial ones may
receive very little nourishment or none at all, which accounts for the
constant exfoliation of cells from the surface skin and other parts.

II. CONNECTIVE TISSUE.

Connective tissue is the most widely distributed tissue in the animal
body. It holds the individual parts of other tissues together; it connects
the various tissues with one another, and at the same time keeps them
separated from one another; it gives firmness to the body as a whole, and
a support for the various organs within it; it forms variously constructed
channels for the distribution of the nutritive material to the various parts

CONNECTIVE TISSUE.

3 11

of the body. With the variety of functions just enumerated there is a
corresponding variety of forms in which connective tissue is found repre-
sented in the body.

We have stated above that connective tissue takes its origin from
the mesoderm. With the advance in the differentiation of the ectoderm
and entoderm the derivatives of these two layers gradually become
separated from one another, and the changes of the mesoderm follows
closely in their steps. It becomes split into two secondary layers, one of
which attaches itself to the derivatives of the ectoderm, ready to serve
them with the above-enumerated functions, and is called parietal meso-
derm, while the other plays the same role in regard to the derivatives of

~ect.

-mesi

Fig. 247. — Diagrams of transverse sections through the body of an embryo of a vertebrate.
It shows the relation of the three germinal layers, ect., Ectoderm; ent., entoderm; mes.,
mesoderm; coe., coelom or body cavity; sk., beginning of skeleton; n., beginning of spinal
cord; g, lumen of the gut. (After Galloway.)

the entoderm, and is called visceral mesoderm. The gap, which remains
between these two secondary layers, is known as the body-cavity or
coelom. The characteristic feature of the mesoderm tissue is that the
cells do not lie side by side and form continuous layers, but are separated
from one another, sometimes quite considerably, and the spaces between
the cells are occupied by a substance somewhat gelatinous in consistency
which is called intercellular substance and is obviously a product of the
cells themselves. The cells are stellated or spindle-shaped, and their
thinned-out processes unite and interlace with one another, forming a
network. The formation of the various forms of connective tissue from
the mesoderm in course of development is due mainly to the differentia-

312 HISTOLOGY.

tion and various chemical changes which that intercellular substance
or matrix undergoes. In view of the fact that the intercellular substance
constitutes the predominating part in connective tissue, it is the fea-
ture upon which the classification of this tissue is based, and we can dis-
tinguish five well-defined characteristic groups: (i) mucous or embryonic
connective tissue, (2) ordinary or fibrous connective tissue, (3) cartilage,
(4) bone, (5) dentin.

Mucous or Embryonic Connective Tissue. — This tissue closely
resembles in its structure the mesoderm tissue in earliest stages of the
embryo, as described above. It consists of a semi-gelatinous, mucoid
matrix, and within it are scattered stellate or spindle-shaped cells, and
here and there thin fibers. The latter present mainly the elongated and
anastomosing processes of the cells, some of them however are un
doubtedly products of the matrix itself, by a process that reminds some-
what of coagulation. This tissue is found at birth in the umbilical cord,
as the so-called jelly of Wharton; in the adult human body it is found in
the pulp of the teeth and in the vitreous humor of the eye.

Ordinary or Fibrous Connective Tissue. — This tissue is present in
the skin and mucous membrane, in the intermuscular tissues, in tendons,
in fascia and aponeuroses, and in the tissues connecting various organs.
It is composed of a meshwork of fine fibers of two kinds. The first,
which makes up the greater part of the tissue, is formed of very fine,
•white, structureless fibers arranged closely in bundles and bands crossing
and intersecting in all directions. The second variety, or the yellow,
elastic fiber, has a much sharper and darker outline, not arranged in
bundles, but is intimately mingled with the white fibers by twisting
around and among its filaments. These are known as the elementary
connective-tissue fibers. The size of the connective-tissue bundles
depends upon the number of elementary fibers present, and by a variation
in the arrangement of the bundles variety in the character of the fibro-
connective tissue is produced in different localities. When the fibrous
connective tissue is formed into an unbroken mass, as in mucous mem-
brane, the minute bundles are collected into smaller or larger groups
(the trabecules:), and these are in turn associated into groups. In the skin
and mucous and serous membranes, the trabecular of the connective-
tissue bundles are separated, and, by crossing and recrossing one another,
form a dense, fan-like structure. In other tissues, as the tendons and
fascia, the bundles are arranged in parallel layers. In the submucous
tissues the connective-tissue fibers are loosely woven, the fibers crossing
and intermingling, with the intervening spaces unusually large, resulting

CARTILAGE. 313

in a loose, flabby tissue. Two varieties of fibrous connective tissue are
distinguished — namely, (a) Compact or fibrillar connective tissue, form-
ing bands of either white fibers, or yellow elastic fibers, or mixed fibers;
and (b) loose or areolar connective, forming a network or reticulum.
Modifications of the loose variety are found represented in adipose tissue
and lymphoid tissue.

The fibrous connective-tissue cells are few in number, of several
varieties, and variously shaped, being flattened, stellate, or apparently
distorted by pressure from surrounding cells or fibrous bundles. In
the mucous membrane the cells are oblong and somewhat flattened,
having many branches which reach out and, uniting with like processes
from neighboring cells, form a network. Other connective-tissue cells
are comparatively larger, oval or rounded in form, granular in appearance,
rich in protoplasm, and are known as plasma-cells. The body of connect-
ive-tissue cells, besides containing a nucleus, frequently contains pigment-
granules; these are known as pigment-cells. These are seldom found in
mucous or serous membranes, being principally confined to the integ-
ument. Fat-globules may also be found in fibrous connective tissue, and
when of considerable size unite and form a rounded cell, called a fat-cell.
Numerous fat-cells uniting, and well supplied with blood-vessels and
nerves, form adipose tissue, or fat. Fat-cells are frequently found in
areolar tissue as well. When fibrous connective tissue is immediately
contiguous to epithelium, it becomes somewhat modified and a new
membrane is formed, called the basement membrane, or membrana propria.
This membrane is a thin, transparent, structureless layer, and, when in
connection with those mucous membranes provided with a layer of vas-
cular fibrocellular tissue, may appear as the formative substance out of
which successive layers of epithelial cells are generated. In the ducts
and glands — for example, the salivary glands — the basement membrane
forms the proper walls of the tubes, and the cells here generated, and
corresponding to the epithelial cells of the coarser mucous membranes,
are known as gland-cells, rather than epithelial cells. This, however,
is a distinction without a perceptible difference, the location and function
as secreting cells being alike in each.

Cartilage. — Cartilage is a semi-opaque, non-vascular tissue, white
in color, and composed of a matrix containing nucleated cells. The
matrix is somewhat elastic and rather dense. The cells are simple in
form, being spheric or slightly inclined to angularity. The variation in
the character of cartilage is due rather to the difference in the character
of the matrix than to the cellular structure, the principal variation in the

314 HISTOLOGY.

cells being in their size. The cells lie in the spaces or lacunae of the
matrix, which they completely fill. Investing the free surface of most
cartilaginous tissue (articular cartilage excepted) is a thin but tough
and firm fibrous membrane — the perichondrium. This membrane is well
supplied with blood-vessels and nerves, and is essential to the growth and
maintenance of the cartilage. There are three varieties of cartilage —
namely, hyaline cartilage, elastic carti'age, and fibrocartilage.

Hyaline cartilage is of a faint pearly-blue color, slightly transparent,
and is found investing the articular ends of the bones — for example,
the condyles of the mandible; also forming the costal and nasal carti-
lages, as well as those of the trachea, bronchi, and a part of the larynx.
Hyaline cartilage is distinguished by a granular or homogeneous matrix.
The cells, which contain a nucleus with nucleoli, are usually grouped
together in patches, and are somewhat irregular in outline, appearing
flattened near the free surface of the tissue in which they are placed, and
inclined to be perpendicular to the surface in the more deeply-seated
portions. The matrix is dimly granular in appearance, resembling
ground glass, and receiving its name from this fact. That part of the
cartilage close to the perichondrium is supplied with cells much smaller
than those occupying the lacunae in the substance of the mass, and the
growth of the cartilage is most active in this part. Lining each lacuna
is a delicate membrane (the capsule), winch primarily is but partly filled
out, but as the cell or cells increase in size, this membrane is carried to
the walls of the lacuna. Articular hyaline cartilage is non-vascular,
being nourished by the blood-vessels of the bone beneath.

Elastic cartilage is of a dull-yellow color, and is sometimes called
yellow cartilage. It is not present in the mouth, but occurs in the exter-
nal ear, in the epiglottis, and in part of the larynx. Its structural compo-
sition is quite similar to hyaline cartilage, but may be distinguished
from it by a network of fine elastic fibers which penetrate the matrix.
The cells are rounded or oval, containing nuclei and nucleoli.

Fibrocartilage is yellowish or milky white in color, and is much more
widely distributed throughout the body than the elastic variety. It is
present in the temporomandibular articulation. Like those previously
described, it is composed of cells and a matrix, the latter being made up
of fibrous connective tissue arranged in bundles, and for this reason it
is scarcely deserving the name of cartilage, only that in other portions
continuous with it cartilage-cells may be found in abundance. Between
the strata of the fibrous bundles are numerous nucleated cells, which are
oval and more or less flattened, and each enveloped in a delicate capsule.

BONE.

315

Cartilage is further classified into two divisions — temporary and
permanent — the former term being applied to that kind of cartilage which
in the fetus and in youth is destined to be converted into bone (for example,
Meckel's cartilage); the latter class including all those cartilages which
are generated as such, and continue to serve in that capacity. Temporary
cartilage closely resembles the hyaline variety, being formed of a matrix
in the lacunae of which the cells are located. These cells, however, are not
grouped together as in hyaline cartilage, but are more uniformly
distributed throughout the matrix.

Bone. — Bone is mainly composed of tricalcium phosphate and
cartilage. The matrix of osseous tissue has a distinguishing feature

Calcified
Matrix

Center of Calcifica-
tion

Enlarged
Lacuna?

Hyaline
Cartilage

Fig. 248. — Developing Bone. X 40

produced by the blending of organic and inorganic substances, resulting
in hardness, solidity, and elasticity. The combination of organic and
inorganic elements in bone is of such a nature that either part may be
removed without destroying the other. The matrix is' composed of the
salts of lime, especially calcium phosphate, and of slender fibrils united
by a cement-substance into bundles of various sizes. The cement-sub-
stance is chiefly composed of insoluble lime-salts, principally carbonates
and phosphates. These two kinds of structure are found to be present
in different parts of the same bone, forming a dense or compact, and a.

316 HISTOLOGY.

spongy or cancellated tissue. The former occur in the shaft of long
bones and in the outer layer of flat or irregularly formed bones. Cancel-
lated bone-substance occurs in the extremities of the long bones and in the
interior of flat and irregular bones. The irregularly formed maxillary
bones give place to both kinds of bony structure; the external layer of
the superior maxillae and the body and rami of the inferior maxilla are
composed of compact tissue, while the interior of these bones and the
condyloid processes of the mandible are spongy or cancellated in their
nature. When examined by the microscope the bony substance is found

Concentric flSL_

Lamella' ~ r r- *■ _«. .^ " -»*» «."" " *■ - %

f •-- ;<* . - ■- > ^\yc > - - »

Haversian M| >. ,~ \ ( C-* < ""* '•» \ »

Canal M| - -■ fc , v ^ _ . ) ^5 , '.-•.>• ? Haversian

*, ' • . ' * v' - ".v ' -* Canal

• * -

Fig. 249. — Transverse Section through Shaft of Long Bone. X 30.

occupied by numerous little spindle-shaped spaces — lacunce. Branching
out from these in various directions are minute canals — canaliculi —
which anastomose with similar canals from neighboring lacunae. In the
maxillary bones no other canals than these may be visible, but if a
transverse section be cut through one of the long bones, an additional
space makes its appearance (Fig. 249).

These spaces are known as the Haversian canals. They are circular
in outline and appear as a center for a small, circular district mapped out
by concentric layers, the lacunae and canaliculi following the same con-
centric plan, and through each other communicating with the Haver-
sian canals. The general direction of the Haversian canals is longitu-

BONE.

317

dinal with the long axis of the long bones, and in the flat or irregular-
shaped bones they are somewhat irregular in formation and ramify in
various directions. In the osseous matrix each lacuna contains a bone-
cell. These are nucleated, protoplasmic cells. In developing bone,
these cells, which do not completely fill the lacunae, are connected by
numerous branches or processes passing through the canaliculi; in older
bone very few processes are observed.

There are two processes by which bone may be prepared for histo-
logical examination, by one method which results in the destruction of

> ' ' A

\ ' \ • \ \

x * \ \ \ * \

Lacuna

Haversian ^k> 1 £

Canal \ \ *t " "• % V^\ \ '

. M * '. A ' \ U • V

Fig. 250. — Longitudinal Section of Long Bone. X 30.

the organic elements, or by another which removes the inorganic ele-
ments. In the former process the organic matter is removed by simply
drying the structures, after which thin sections may be prepared and care-
fully examined under the microscope, when the Haversian canals, lacunae,
and canaliculi will be seen forming a complete concentric network. In
the latter method the inorganic substance is removed by immersing a
fresh bone in dilute picric acid, C 6 H 2 (N0 2 ) 3 OH, which readily decalcifies
it, and when properly prepared sections are placed under the microscope
the organic contents of the lacunae and canaliculi alone are visible.

The concentric laminae of bone is riveted together by numerous
delicate rods of processes named Sharpey's fibers, these delicate fibers
passing through the laminae to perform this office.

318 HISTOLOGY.

Periosteum and Bone-marrow. — The interstices of spongy
bone are filled, with a soft mass — the bone-marrow — and the external
surface of the bone is covered by a fibrous membrane — the periosteum.
This membrane is absent where bones are joined to each other by liga-
ment or cartilage, and over articular surfaces. The periosteum is a
compact connective-tissue membrane. It consists of two layers: an
outer, fibrous layer rich in blood-vessels, which forms the connection with
adjacent structures; an inner or osteogenetic layer containing few blood-
vessels, loose in texture, but rich in elastic fibers and spheric connective-
tissue cells, with oval nuclei. These are the formative cells of bone and
are called osteoblasts. These cells appear in the lower strata of the inner
layer, or the layer in contact with the bone, and are especially numerous
during the period of development. Through the blood-vessels of the
bone the marrow, internally, is placed in communication with the perios-
teum externally; small branches given off from the numerous arteries and
veins of the periosteum enter the Haversian canals, upon which they pass
to the canaliculi, thus communicating with the blood-vessels of the
marrow. In like manner numerous nerves enter the substance of the
bone, first passing into the Haversian canals, after which they become
closely associated with the minute blood-vessels and are distributed
to the periosteum and bone-marrow. The bone-marrow, besides fill-
ing the interstices of the spongy substance, is also found occupying
the central cavity of long bones, and in the larger Haversian ca-
nals. The marrow is of two varieties, distinguished by its color, being
either red or yellow. Red marrow is found in the flat bones (including
the maxillae), the vertebras, and ribs, while yellow marrow occurs in the
long bones of the extremities. Red marrow is composed of a delicate
connective-tissue network supporting, besides the marrow-cells, a few
fat-cells and giant-cells. In the long bones the yellow marrow is sur-
rounded by a connective-tissue membrane lining the medullary canals.
Marrow-cells and giant-cells are present in abundance. Marrow is very
vascular and contains many osteoblasts.

Dentin. — This structure, as well as cementum, which in many
particulars closely resembles bone, will be fully considered in connection
with the histology of the tissues of the teeth.

III. MUSCULAR TISSUE.

Muscular tissue consists of elongated or fiber-cells and according to
the structure of these fibers it is divided into three classes — non- striated,
striated and cardiac.

NON-STRIATED MUSCULAR TISSUE.

3 J 9

Non-striated, Smooth, or Involuntary Muscular Tissue. — This
tissue consists of contractile fiber-cells which are elongated, spindle-
shaped, and cylindric, with exceedingly elongated extremities, which
become shorter and thicker through contraction. They are quite variable
in length (i/io to 1/450 of an inch), and are composed of a pale, homo-
geneous-looking protoplasm, each inclosing an elongated or rod-shaped
nucleus, which is flattened if the cell is so formed. The muscular fibers
are firmly bound together by a cement-substance, forming fasciculi,
which in turn are collected into strata or membranes, which may be dis-

FiG. 251. — Transverse Section of Striated or Voluntary Muscular Tissue. X 40.

posed parallel, or crossing and recrossing, forming an intricate network.
The connective-tissue septa provide a passageway for the larger blood-
vessels, while the capillaries penetrate the fasciculi forming a compli-
cated network with oblong meshes. Involuntary muscular tissue is not
found in the mouth except in the ducts of the salivary glands. They form
the main constituent part of the middle layer or coat of blood-vessels
and are particularly abundant in arteries.

Striated or Voluntary Muscular Tissue. — Striated muscular
tissue is composed of long, cylindric fibers, which are regularly transversely
striated. In most instances their extremities are attached to bones by
means of tendons, as, for example, the cheek- and lip-muscles. The

3 20

HISTOLOGY.

fibers are grouped together by fibrous connective tissue into various sized
bundles, forming fasciculi. There is much variation in the length of the
fibers composing the fasciculi in different muscles. In most instances
the fasciculi which serve to make up the bundles of a single muscle con-
tinue parallel with one another throughout their length. Surrounding the
whole muscle is a layer of connective tissue called epimysium; this pene-
trates between the individual bundles and forms a covering for each one
of them, which is called the perimysium, and passing from this into the
substance of the bundle is a still more delicate connective tissue, the

Fig. 252. — Striated or Voluntary Muscular Tissue. X 40

endomysium, which separates the individual fibers from one another.
The former structure carries the larger blood-vessels and nerve-fibers,
while the latter supports the capillaries.

Each muscular bundle may again be divided into smaller bundles,
which in turn are ensheathed in a similar manner and further divisible,
so continuing, until the primitive fasciculi, or so-called muscular fiber,
is reached. Striped muscular fiber consists of a structureless, elastic
sheath, the sarcolemma, which structure represents the cell- membrane,
and closely invests a number of filaments or fibrils. Besides the fibrillar,
there is contained within this fine, structureless, transparent membrane

STRIATED OR VOLUNTARY MUSCULAR TISSUE. 32 1

the sarcoplasm, a faintly granular substance resembling protoplasm,
but not identical with it. This substance serves in the capacity of a
matrix for the fibrillae. The nucleus is found beneath the sarcolemma.
The fibrillae are arranged parallel to one another, being supported by
the sarcoplasm. It will thus be seen that each fiber of a striated
muscle comprises the sarcolemma, the muscle-nuclei, the fibrillae, and,
finally, the sarcoplasm, filling all the interstices, first between the
fibrillae of each muscle-column, between the columns of each group,
and between the groups themselves. The disposition of the sarcoplasm
may be most favorably studied by a cross-section through the fibers,
appearing as a delicate but clear network, within the meshes of which
are the muscle-columns. Striated muscular fibers are usually tapering
off and becoming thinner toward their extremities. In rare instances
they are branched at their ends. This condition is present in the
tongue, the extremities of the fibers passing transversely into the oral
mucous membrane, where they become further subdivided. The striated
or voluntary muscles make up the muscular tissues of the lips, cheeks,
tongue, and soft palate.

Cardiac or Involuntary and Striated Muscular Tissue. — This
tissue is found only in the heart. The fibers are short, striated and have
the nucleus in the center.

IV. NERVOUS TISSUES.

Until within recent times it has been stated that the nervous tissue
consists of two histologic elements known as nerve-cell and nerve-fiber;
that these two elements differed not only in their mode of origin, but in
their structure and physiologic endowments. At the present time it
is believed that the entire nervous system consists of an infinite number
of definite independent morphologic units, which, through having a com-
mon origin and a similarity of structure, have, nevertheless, different
functions in different parts of the body. This neurologic unit has been
termed the neuron, and, as represented schematically in figure 253, may be
said to consist of: First, the nerve-cell, or neurocyte; second, nerve-
process, or axon; third, the end-tufts, or terminal branches. Each of
these three main portions of the neuron presents a variety of secondary
features which are related to their functional activities.

The Nerve-cells, or Neurocyte. — The nerve-cells are found in the
cortex of the brain, in the interior of the spinal cord, in the various gang-
lia of the cerebrospinal and sympathetic nervous systems, and in the
organs of special sense. All neurocytes are the modified descendants of
21

322

HISTOLOGY.

Nerve-
cell

^Dendrites

Naked
Axis-
cylin-
der

Axis-
cylin-
der En-
veloped __,

by-
Medul-
lary
Sub-
stance

Collateral Branch

independent oval or pear-shaped cells (the neuroblasts), originating from

the epithelial cells which form the
medullary tube. The neurocyte is
at first smooth, devoid of processes,
and endowed with ameboid move-
ment. In the course of develop-
ment the cells project a greater or
less number of processes and as-
sume a variety of shapes and sizes,
in accordance with variations in
functions; thus, the cells may be
spheroid, pyramidal, spindle-
shaped, stellate, etc. The body of
the cell consists of a protoplasmic
basis, more or less granular, con-
taining a well-defined nucleus and
nucleolus. A centrosoma has also
been found in the nerve-cell in
many situations. There is no evi-
dence, however, of the existence of
a cell-membrane. From the body
of the neurocyte there arises one
or more protoplasmic processes,
which, passing outward in various
directions, divide and subdivide
into a greater or less number of
branches, which are collectively
known as dendrites or dendrons.
The ultimate subdivisions and
terminations of a dendrite, though
forming an intricate feltwork,
always end free, never anastomos-
ing with one another. Arising from
the cell-body, the dendrites resemble
in appearance and structure the
cell-protoplasm, or cytoplasm. In
the cortex of the cerebrum and in
the cortex of the cerebellum the
dendrites are characterized by

Axis-
cylin-
der En-
veloped
by both
Neuril-
emma "
and the \
Medul-
lary
Sub-
stance

.'Terminal Branches

Fig. 253. — Diagram of a Neuron.
Stohr's "Histology.")

(Frctn

short, lateral projections known

NERVOUS TISSUES. 323

as lateral buds or gemmules, which impart to the dendrite a feathery
appearance.

The Axon, or Nerve-process. — The axon is the first outgrowth of the
protoplasm of the neuroblast, but with the development of the neurocyte
it becomes so differentiated from the dendrites that it can be readily
distinguished from them. It usually arises from a cone-shaped projection
of the cell-body, though occasionally it arises from a dendrite itself. It is
characterized by a short, regular outline and a hyaline appearance. The
majority of the cells, especially in the mammalia, possess but one axon,
though in the developing ganglion-cells of the spinal nerves two distinct
axons are present. In their subsequent development the two axons appear
to blend together to form but a single axon, which, at a short distance from
the cell, again divides into two branches, which pursue opposite directions,
one passing directly into the spinal cord, the other toward the periphery.
The axon may continue as an individual structure for an indefinite dis-
tance, varying from a few millimeters to ioo cm. In the former instance
the axon, at a distance of a few millimeters from the cell, breaks up into
a number of branches, which form an intricate feltwork in the neighbor-
hood of the cell. This type of cell is not widely distributed, being
confined largely to the cerebellum. In its course the axon, more espe-
cially in the central nervous system, gives off a number of side-branches or
collaterals, which do not differ from the axon itself, either in structure or
appearance. The axon of the peripheral nerves, especially the spinal
nerves, are devoid of collaterals throughout their extent, except, perhaps,
in the immediate neighborhood of the cell. The more or less elongated
axon becomes inclosed at a short distance form the cell with a thick
layer of fatty material, forming a medulla or myelin, inclosed by a deli-
cate cellular sheath (the neurilemma), and thus constitutes what is com-
monly known as a medullated nerve-fiber. In the central nervous sys-
tem the neurilemma is frequently wanting. In the sympathetic system
the myelin is wanting, though the axon is inclosed by a delicate sheath
resembling the neurilemma, thus constituting a non-medullated nerve-
fiber. The collateral branches are provided with similiar investments.

The End Tufts, or Arborizations. — Each axon, as it approaches
its final termination, breaks up into a number of branches, which vary
in complexity and appearance in different regions. They are always
free from any medullary investment, and appear to be formed by the
splitting of the axon into a number of fine filaments, which remain inde-
pendent of one another. In peripheral organs, as muscles, glands, and
blood-vessels, the tufts are in direct organic connection. In the cen

3 2 4

HISTOLOGY.

tral nervous system the end-tufts are in more or less intimate relation
with the dendrites of other neurons.

Nerves. — Nerves are to be regarded, therefore, as groups of axons,
with their medullary investments connecting the peripheral organ with
the central nervous system.

The nerves are arranged in two great systems — the cerebrospinal
and the sympathetic. In the cerebrospinal nerves the conducting
media — the nerve fibers — are arranged in parallel or interlacing bundles,
and these are further grouped into nerve-branches or nerve-trunks.
The bundles are connected by intervening fibrous connective tissue

Shea'

Endoneuriur

Medullan
Sheath

Perineurium

Fig 254. — Portion of Transverse Section of Human Median Nerve. X 200.

(the epineurium), and through this tissue the principal blood-vessels
ramify to supply the nerve-trunks, together with a plexus of lymphatics
and numerous fat-cells and plasma-cells.

The size of the nerve-bundles, or funiculi, is regulated according
to the size and number of nerve-fibers which they contain. Investing
each funiculus, or primary bundle of nerve-fibers, is a connective-tissue
sheath — the perineurium. The fibers composing this sheath are arranged
in lamellae, being separated from one another by lymph-spaces vari-
able in size, through which communication is afforded the lymphatics of
the epineurium. Within the bundles the nerve-fibers are held together

NERVES.

3 2 5

by fibrous connective-tissue — the endoneurium. The epineurium holds
together and envelops the several funiculi of the nerve-trunk, the peri-
neurium investing each funiculus, or primary bundle of nerve-fibers,
and the endoneurium extending among and around the individual fibers.
Nerve-fibers are divided into two classes — which classification is depen-
dent upon the presence or absence of a medullary sheath or covering —
into the medullated or white, and the non-medullated or gray. The
medullary sheath, or white substance of Schwann, is a bright, fatty sub-
stance (the myelin) surrounding the axon, or axis-cylinder, the conducting
or central part of a nerve-fiber. Between the medullary sheath and the
axis-cylinder there is present a small amount of albuminous fluid. Closely

rv- •

Perineurium ' .',i/y^s> -*' J?*

Fig. 255. — Transverse Section, Bundles of Nerve-fibers, Human Median Nerve. X 30.

surrounding the medullary sheath, and forming the outer boundary
of the nerve-fiber, is the neurilemma, or sheath of Schwann. Between
this delicate, structureless membrane and the medulla there are placed
at intervals oblong nuclei, surrounded by protoplasm; these are the
nerve-corpuscles. Besides the division of nerve-fiber into medullated
and non-medullated, each division is susceptible of further subdivision,
dependent upon the presence or absence of the neurilemma. Non-
medullated nerve-fibers without a neurilemma are composed of an axis-
cylinder only; they are cylindric or band-like in form, transparent, and
show faint, longitudinal striations. Non-medullated nerve-fibers with a
neurilemma are composed of an axis-cylinder surrounded by a neurilemma,
and are homogeneous throughout their extent.

326 HISTOLOGY.

Medullated nerve-fibers are those which are partly, but never entirely,
invested by a medullary sheath. They may or may not possess a
neurilemma; in the former instance they consist of an axis-cylinder and a
medullary sheath only. The axis-cylinder, or essential part of the nerve-
fiber, is cylindric or band-like, occasionally exhibiting a delicate, longitu-
dinal striation, which appearance is due to its being composed of a primi-
tive fibrilke.

The nerve-cells or ganglia-cells are found in the ganglia as well as
along the course of the nerves. They are composed of granular or
faintly striated protoplasm, inclosing a characteristic nucleus within which
is a nucleolus. They differ greatly in form as well as in size, the spheric,
spindle-shaped, and irregularly stellate forms being the most common.
In the latter numerous processes are given off, forming the stellate out-
lines. The cells are variously named, according to the number of proc-
esses. If one process is present, the cell is termed a unipolar cell; if
two, a bipolar; and if a number of processes exist, they are named multi-
polar. The processes are of two varieties — the axis-cylinder process
and the branched protoplasmic process. The various forms are most
readily distinguished in the multipolar cells. The axis-cylinder process
is readily characterized by its hyaline appearance and unbroken outline.
The protoplasmic processes are thicker, granular, and striated.

V. BLOOD AND LYMPH.

It is rather difficult to conceive that blood and lymph can be spoken
of as representing an animal tissue. The conception of a tissue generally
carries with it the idea of some well-defined, stationary texture; blood
and lymph, on the contrary, present fluids, which uninterruptedly stream
along within closed channels — the blood- and lymph-vessels. A
closer study of the development and structure of blood and lymph, and a
little reflection in regard to their physiological role in the economy, shows,
however, that, while they present certain peculiarities, it is nevertheless
justified to classify them as a tissue.

Development. — Blood and lymph, as well as blood- and lymph-
vessels, originate in the mesoblast. Some of the cells of the latter become
more closely united in groups arranged in form of cords. These gradually
become transformed into tubes, the walls of which consist of a single
layer of cells arranged in epithelial-like fashion, and are called endothe-
lium. Other cells, while remaining separated from one another, become
inclosed within the channels and give rise to the red and white blood-

BLOOD AND LYMPH. 327

corpuscles. The intercellular substance between them, in the meantime,
becomes differentiated as a fluid called blood- plasm, thus allowing the cells
free floating within the channels.

Structure. — The microscopic examination reveals the fact that
blood and lymph consists of a large number of cells. The cells do not
however lie side by side and are held together as in epithelial tissue; they
are neither separated from one another, and at the same time fixed in a
relative position to each other by means of processes and a matrix of
various consistency, like connective tissue. They are suspended in a
substance which normally is a fluid, but when however examined in a
state of coagulation, it has a more gelatinous consistency and contains
fibers, thus reminding of the intercellular substance in connective tissues.

Physiology. — Blood and lymph presents the medium which carries
nutrient and building material to all parts of the body, and in exchange
receives and carries away the waste products. To fulfill this function,
their active constituents, the cells, must be supplied with facilities for
reaching all parts of the body, and this is obviously accomplished through
the intercellular substance being a fluid.

CHAPTER III.

The Mucous Membrane of the Mouth; of the Lips; of the Cheeks;
of the Gums; of the Roof of the Mouth, Hard and Soft Palate ;
of the Floor of the Mouth; the Tongue.

HISTOLOGY OF THE TISSUES OF THE MOUTH.

Mucous Membrane of the Mouth. The mucous membrane lining
the cavity of the mouth consi>ts of two parts — the epithelium and the
tunica propria; beneath the latter, and forming the deeper part of the
mucous membrane, is the submucosa.

^fli

layer
of Cells

Infant Lay-
er of Cells

Connective
Tissue

Fig. 256. — Vertii .1] S( 1 lion, Mucous Membrane of the Mouth, Human Embryo. X 150.

The epithelium of the mouth is a thick, stratified, squamous epithe-
lium, the most superficial cells being scale-like or horn-like. The cells
are arranged similar to those in the epiderm, the lower layers are
columnar in form, and contain very little pigment.

328

MUCOUS MEMBRANE OF THE LIPS. 329

The tunica propria is a somewhat dense feltwork of interlacing connect-
ive-tissue bundles, interspersed with elastic fibers. The tunica propria
penetrates the epithelium in the form of cylindric or conic papilla?, which
differ in length with the variation in the thickness of the epithelium.
As the mucosa is usually thickest in the lips, gums, soft palate, and uvula,
accordingly the papilla? are of the greatest length in these parts. The
tunica propria passes into the submucosa so gradually that a positive line
of demarcation cannot be established.

The submucosa consists of a bundle of fibrous connective tissue with
but few elastic fibers. This structure is somewhat loose in texture and
is loosely attached to the underlying periosteum. Over the major portion
of the gums and the entire hard palate the submucosa is attached to the
bones of the mouth through the medium of their periosteal covering.
It is in this loosely constructed tissue that the glands of the mucous mem-
brane are situated. These are for the most part branched, tubular, mucous
glands. Besides adipose tissue in the form of groups of fat-cells, striped
muscular tissue is present in the submucosa. In some parts of the mouth
this tissue forms a conspicuous portion — namely, in the sphincter muscle
of the lips (orbicularis oris) ; also in the soft palate, uvula, and pillars of
the fauces.

The blood-supply to the mucous membrane of the mouth is principally
distributed in two systems, the larger vessels to the submucosa and the
capillaries to the tunica propria. The larger vessels break up and send
a dense network of capillaries through its substance and to the numerous
papilla? which extend into the epithelium. Numerous veins ramify
through the superficial part of the tunica propria. The lymphatics form
two networks, the submucosa giving place to the coarser vessels, while
the fine parts are distributed to the tunica propria.

Nerve-supply to the Mucous Membrane of the Mouth. — In the sub-
mucosa the medullated nerve-fibers form a wide-meshed reticulum, from
which numerous primitive fibrilla? pass to the tunica propria, where they
terminate or continue as non-medullated nerve-fibers, and penetrate the
papilla? of the epithelium, forming networks.

Mucous Membrane of the Lips. — Beginning as a direct continuation
of the integument or external covering of the lips, the labial covering,
including the integument, may be divided into three parts — namely, a
cutaneous portion (best described in this connection), a transitory portion,
and a mucomembranous portion.

The cutaneous portion, covered by a thin epidermis, consists of a
double layer of somewhat flattened epithelium. Immediately beneath

33© HISTOLOGY.

this is a thin, cellular, mucous layer, the cells composing it being spheroid
in form, and containing nuclei which are proportionately large. Sub-
jacent to this is the cutis, composed of fasciculi of fibers intersecting
and closely woven together, the principal fibers passing toward the free
border of the mucous membrane covering the contiguous surface of the
lip. These fibers are for the most part connective-tissue fibers, intermingled
with elastic-tissue fibers. Numerous small, vascular papillae are found
upon the surface of the cutis; these are cylindric or conic in form, and
project for some distance into the rete mucosa — the lower layers of
living cells of the epidermis. Equally distributed at various depths in
this tissue are numerous hair- and sebaceous follicles. The general
direction of the hair-follicles in the upper lip is downward, while those
occupying the lower lip are turned upward. Other than the distinction
noted by the difference in color of the parts, the cutaneous portion may
be distinguished from the transitional mucous membrane by the absence
of hair-follicles and sebaceous glands in the latter.

The transitional portion of the mucous membrane of the lips is out-
lined externally by the outer border of the red portion of the lips, and
internally by that prominent part of the labial convexity which comes in
contact with the opposing labial fold, leaving the transitional portion
exposed to view when the lips are in occlusion. The epithelial layer
of this surface does not begin where the hair-follicles cease to exist, a
slight interspace appearing upon the cutaneous portion which is devoid
of these follicles. At its line of beginning the transitional portion of the
labial mucous membrane is quite thin, but rapidly increases in thickness
in passing toward the mucomembranous portion. Superficially the
cells are much flattened, closely associated with one another, and devoid of
nuclei. The cells of the middle and deeper layers are oblong or spheric,
and provided with irregularly shaped nuclei. The chief fibrous tissues
of the transitional portion, which are thinnest at the point where the
hair-follicles cease to exist, are united into flexiform fasciculi, which are
separated at various points to give passage to numerous minute blood-
vessels. The fibrous tissues increase in thickness as the mucomembran-
ous portion is approached. Numerous thin and somewhat elongated
papillae are distributed over the surface of the transitional portion.

The mucomembranous portion of the mucous membrane of the lips
includes all that portion covering the labial folds within the mouth,
beginning at the line of occlusion on the contiguous surface and extend-
ing to the gums. The epithelium is much thicker than that previously
described, and presents the characteristic layers common to stratified,

MUCOUS MEMBRANE OF THE CHEEKS.

33 1

squamous epithelium. Superficially the cells are flattened and tubular,
provided with nuclei of similar form. In the middle layer the cells are
flattened and oblong, followed in the deeper layer by irregularly formed
nucleated cells. A variety of fibers make up the structure— one class
fine in texture and united into fasciculi, intermingled with elastic fibers,
together with another set of coarse, strongly looped fibers. Whenever
the fibers of the tunica propria assume a definite general direction, they
are horizontal, passing from right to left and encircling the oral aperture.
The tunica propria is beset with numerous conic papillae which project

Embryonal

Mucous
Membrane

Buccal Cavity

)

Fig. 257. — Vertical Transverse Section through Head of Human Embryo, about the Sixth
Week, showing Single Buccal Cavity. X 30.

into the epithelium; these are longest where the epithelium is thickest.
The mucous membrane forming the labial frena is covered by an epithelial
layer which is much thinner than that distributed to other parts of the lips.
The fibers in these are irregularly distributed, and the papillae are small
and not so numerous. The coronary arteries and their accompanying
veins course through the lips near the junction of the transitional with
the mucomembranous portion of the mucous membrane.

Mucous Membrane of the Cheeks. — The mucous membrane of
the cheeks presents but little variation in its structure from that of the
mucomembranous portion of the labial mucous membrane. The buccal
epithelium is the same in structure and thickness as that of the lips,

33 2 HISTOLOGY.

excepting the disposition of cells in the middle layer, where they are greater
in number and more closely associated, being somewhat distorted by con-
tact. The papillae, which project from the mucosa into the epithelium,
are somewhat broad at their base, with elongated extremities, the height
of which is quite variable, in some instances penetrating well into the
epithelium, at others merely entering its deeper layer. At the anterior
portion of the cheek, or that in the region of the angle of the mouth,

Fig 258. — Section through the Mucous Membrane of the Cheek, showing the Papilla; of

the Mucosa in Transverse Section.

the mucous membrane, by its submucous portion, is in immediate contact
with the fibers of the buccinator muscle, and throughout the entire surface
of the cheek it is closely associated with this muscle. The membrana
propria is dense immediately beneath the epithelium, but as the buccinator
is approached it becomes much less so.

Mucous Membrane of the Gums. — The mucous membrane cover-
ing these parts is, on account of the numerous tendinous fasciculi which
enter into its construction, extremely dense and tough, these character-
istics being more strongly manifest here than in any other portion of the

MUCOUS MEMBRANE OF THE GUMS.

333

oral mucous membrane. These qualities are especially pronounced
about the gingival margins and over the major portion of the
alveolar walls, being closely bound down to the bone by direct pro-
longations of the tendinous fasciculi of the periosteum which penetrate
the membrane. As the gingival mucous membrane passes into that of
the lips and cheeks it gradually becomes less dense. The epithelium
of the mucous membrane of the gums is composed of lamina of tessellated

Epithe-
lium

Papilla

Fig. 259. — Section through the Gums, showing Epithelium and Basement Membrane.

and ribbed cells. The superficial cells are the flattened cells of pave-
ment epithelium; subjacent to this they become thicker and deeply ribbed,
while the deepest cells are conic or cylindric with conic extremities. The
tissue composing the tunica propria is made up of flattened fasciculi of
connective tissue, the fibers of which run parallel with one another.
Numerous elastic fibers are also present. Three sets of fibers are to be
distinguished in the mucous membrane of the gums — those which run
vertically, those which pass in a horizontal direction, and those which
radiate or are distributed fan-like. Of the first named, the fibers extend

334 HISTOLOGY.

from above downward; in the second class they pass from right to left
parallel with the surface; the third class, including those fibers which are
reflected from the alveolodental membrane, are distributed in fasciculi
about the margins of the alveoli.

Mucous Membrane of the Roof of the Mouth. — Hard Palate.—
The mucous membrane covering the hard palate is, in very many respects,
dissimilar to that surrounding the necks of the teeth and forming the
palatogingival margins. Like the mucous membrane of the gums, that
overlying the hard palate is dense and tough. The papilla? of the tunica
propria, which penetrate the epithelium, are not so numerous as those
upon the gums. In the posterior third of the hard palate they are some-
what more numerous and generally a little more prominent than those in
the anterior portion. In the median raphe and over the rugae, the papilla?
are especially sparingly distributed. The epithelium is of the pavement
variety, somewhat thinner in front than behind, the cells being more
freely distributed at some points than at others. The mucous membrane
of the hard palate is less in thickness anteriorly than posteriorly. The
distribution of the fibers is such that they radiate from the alveolar borders
toward the center of the palate, the anterior fibers passing obliquely
backward, while those from the lateral walls pass parallel with one
another to the median line. For the most part the fibers are broad and
form a plexus between the epithelium and the submucous tissue. The
submucous tissue is sparingly distributed over the central portion of the
hard palate, but laterally is somewhat more abundant, containing a few
fat-cells.

Soft Palate, Uvula, and Fauces. — Passing backward from the posterior
margin of the hard palate, the mucous membrane overlies the fibrous
aponeurosis of the soft palate and its median and lateral prolongations —
the uvula and pillars of the fauces. The epithelium is of the laminated
pavement variety, with the deeper cells larger than those placed super-
ficially. The substance of the mucous membrane is composed of fasciculi
of connective tissue, intermingled with a plexus of elastic fibers. The
fibers are distributed in three principal directions — from side to side,
or horizontally, longitudinally, and obliquely. The oblique fibers are
instrumental in forming the submucous tissue of both the soft palate and
uvula. Numerous conic papillae project from the tunica propria into the
epithelium; these are larger and more numerous on the uvula than on the
soft palate. The tunica propria is somewhat variable in thickness to
accommodate the glands, which are more or less numerous, and present
in greater numbers in one instance than in another. In general the

MUCOUS MEMBRANE OF THE FLOOR OF THE MOUTH.

335

membrane as a whole is thinnest along the margin of the hard palate,
gradually increasing in thickness as the free border is approached. The
folds of mucous membrane forming the pillars of the fauces present no
peculiarity differing from that of the soft palate, save a more generous
supply of elastic fibers.

Mucous Membrane of the Floor of the Mouth.

The Tongue.— The entire unattached surface of the tongue is covered
by a reflection of the mucous membrane of the floor of the mouth. In this
organ the general structure of the mucous membrane does not vary from
that of other oral mucous membrane, being composed of an epithelium,
a tunica propria, and a submucosa. The mucous membrane covering

Epithelium

Tunica
Propria

!iV

Fig. 260. — Longitudinal Section through Mucous Membrane of the Human Tongue. X 20.

the dorsum of the tongue presents special characteristics, which differ
from that of the under surface and the floor of the mouth in general.
In the former location the papillary elevations of the tunica propria are
conspicuously developed, and with their covering of stratified, scaly
epithelium cause the peculiar furred appearance. Three classes of
papillae are distinguished, named, in accordance with their form, filiform
papillae, fungiform papillae, and circumvallate papillae.

The filiform papilla, which are very numerous over the entire dorsum
and sides of the tongue, are conic and frequently prolonged into numerous
horn-like processes, known as secondary papillae. As elevations from
the tunica propria they are composed of well-defined fibrillated tissue,

336

HISTOLOGY.

intermingled with numerous elastic fibers. The pavement epithelial cells
are found overlapping one another, and provided with processes which
project beyond the papilla?.

The fungiform papillce are also distributed over the entire dorsum
and sides of the tongue, but are somewhat less numerous than the filiform
variety. They appear as well-defined elevations, and are connected
with the tunica propria by a constricted portion or neck. The entire
free or rounded surface of these papilla? is beset with secondary papillae.
The epithelium is slightly thinner than that over the filiform papillae, this
being the principal distinguishing feature. The numerous capillaries
produce a rich red color, plainly observable through the transparent

Fig. 261. — Longitudinal Section of the Mucous Membrane of the Human Tongue, showing
the Fungiform and Secondary Papillae. X 80.

epithelium. Connective-tissue bundles make up the bulk of these
papillae, few elastic fibers being present.

The circumvallate papillce are placed on the posterior portion of the
dorsum of the tongue, and are few in number (eight to sixteen) . They are
much larger than those already described, and in general resemble modified
fungiform papilla?. They are flattened and broad, and differ from the
fungiform by having a circular furrow or wall surrounding them. Second-
ary papillae are present on the free surface only, the sides, and in some
instances the walls surrounding them, being occupied by the end organs
of the special sense of taste — the taste-buds. Other taste-buds are found
upon the lateral margins of the tongue posteriorly, nestled in a group of

BLOOD-SUPPLY TO THE MUCOUS MEMBRANE OF THE MOUTH. 337

parallel folds of mucous membrane — the papillae foliata. The connective
tissue within these papillae is similar to that in the fungiform papillae.
On other parts of the tongue, or those portions not occupied by these
specially constructed papillae, the epithelium is similar to that in other
parts of the mouth. The tunica propria is less in thickness in and about
the tip of the tongue, and is intimately connected with the subjacent
muscular structure. As the root of the organ is approached, the tunica
propria becomes thicker and more dense. The submucosa is especially

Fig. 262. — Section through Epithelium, Near the Tip of the Tongue. X 40.

intimately connected with the underlying parts at the margins and tip
of the tongue. The extreme portion of the root of the tongue has its
mucous membrane particularly modified by a special aggregation of
adenoid tissue — developed lymph-nodules. These are large and readily
perceptible to the naked eye. They are provided with a central opening,
which dips down into a well-defined vault or crypt, which is lined by a
reflection of the stratified oral epithelium.

Blood-supply to the Mucous Membrane of the Mouth. — The oral
mucous membrane derives its supply of blood from numerous branches
of the external carotid artery — namely, the superior and inferior coro-
nary, buccal, lingual, transverse facial, pterygopalatine, and the alveolar.
Entering the submucosa, the minute terminal branches of these arteries
are distributed parallel to the surface, and by anastomosis form plexuses
from which other minute branches are given off to supply the papillae of
the tunica propria. After coursing through the papillae the blood is
22

338 HISTOLOGY.

discharged into a similar venous plexus, and thus conveyed from the
parts. In a like manner the mucous membrane and papilke of the
tongue are supplied, branches of the lingual artery conveying the blood
to the parts. The dorsalis linguae supplies the mucous membrane of the
dorsum of the tongue and pillars of the fauces, while the ranine artery
by its minute branches supplies the remaining mucous membrane. Each
papilla is entered by two or more arterial terminals, which divide, anas-
tomose, and finally send off capillary branches to the secondary papilla?.
Nerve-supply to the Mucous Membrane of the Mouth. — The
distribution of the nerve-fibers to the oral mucous membrane is approxi-
mately similar in all parts. The fibers, which are of the medullated
variety, are first distributed to the submucosa, forming a wide-meshed
reticulum. From this fibers are given off to the tunica propria, terminat-
ing in end-bulbs, or, after losing their medullary sheath, are distributed
to the epithelium, where their free extremities lie between the epithelial
cells. The nerves of the mucous membrane of the tongue (the glosso-
pharyngeal and lingual branch of the fifth) may have their endings similar
to those in other parts of the mouth, or they may be intimately associated
with the taste-buds.

Literature.

Legros and Magitot, 1880.

Klein, "Structure of the Oral Lips," 1868.

Sebastian, "Anatomy and Physiology of the Labial Glands," 1842.

Kolliker, "Mikroskopische Anatomic"

Kirke, "Physiology."

Strieker, "Human and Comparative Histology."

Stohr, "Text-book of Histology," 1896.

CHAPTER IV.

Glands and Ducts of the Mouth; of the Lips; of the Cheeks; of
the Hard and Soft Palates; of the Tongue. — The Salivary
Glands.

GLANDS AND DUCTS.

Glands of the Mouth. — The glands of the mouth, like the glands
of other parts of the body, are composed almost entirely of epithelium,
and may, therefore, be classed with the epithelial tissues. Glands exist
in two principal forms — tubular and saccular (alveolar). The former
occur either singly or in groups, and are further subdivided into simple
tubular and compound tubular glands. A like condition is present in
the saccular glands and similar terms are employed to qualify them —
simple saccular glands and compound saccular glands.

A simple tubular gland is one composed mainly of a simple tube-
like structure; a compound tubular gland is one composed of a number of
smaller tubes emptying into a single duct.

A simple saccular gland is one formed by a sacculation of serous or
mucous membrane into a single, simple sac, or by branched saccules
having an excretory duct (alveolar system) ; a compound saccular gland is
composed of a combination of branched saccules.

In the larger glands a sheath is formed by the surrounding connective
tissue, from which numerous septa are given off to the interior of the
gland, dividing it into compartments varying in size. These are known
as gland-lobules. The connective-tissue walls of the gland-lobules carry
the larger blood-vessels and nerves. Most glands are divided into two
essential parts — the gland-follicle and the excretory duct — the former
being specialized for the secretory function, while the latter, by com-
municating with the surface, conveys the secreted substance to that point.

The gland-follicles are composed of a layer of gland-cells, usually
simple in character, surrounding the follicular walls. External to these
is a specially modified connective tissue, forming the basement membrane,
or membrana propria. The appearance of the gland-cells and their
nuclei is continually changing, being thus influenced by their functional
activity.

339

34°

HISTOLOGY.

The excretory ducts consist of a wall of connective tissue and elastic
fibers, lined by a columnar epithelium, either simple or stratified. In
some instances the arrangement of the excretory ducts is much compli-
cated, being divided into secretory tubes, which in turn are subdivided
into smaller tubules — intercalated tubes.

Fig. 263. — Section through the Glandular Tissue of the Tongue, x 40.

The Glands of the Lips (Labial Glands). — The glands of the lips are
situated in the submucosa, and arc first observed immediately within the
line of labial occlusion, at which point the thickness of the epithelium
becomes somewhat definite and general. These glands are variable in
size, but all are sufficiently large to be observed without the aid of the
microscope. They are of the compound tubular variety, and communi-
cate with the surface through an excretory duct, which throughout the
greater part of its extent is lined with stratified, scaly epithelium. In
passing from the surface toward the gland-follicle, the main duct takes a
spiral course obliquely through the tunica propria, and upon reaching
the submucosa gives off numerous branches and twigs which terminate
in the individual acini. The larger branches from the main duct are

THE GLANDS OF THE CHEEKS. 34.I

lined with stratified squamous epithelium, while the smaller twigs are
provided with columnar epithelium. In many instances the main
excretory duct, in its passage through the tunica propria, receives the
principal duct from small accessory ducts. The framework of the labial
glands is formed by the flexiform tissue composed of fasciculi of the fine
connective-tissue fibers belonging to the submucous layer, together with
delicate, coiled elastic fibers. This framework gives support to a minute
system of capillaries and small nerve-fibers supplying the acini. The
acini are so arranged that those belonging to a large duct are united
into a lobule by the submucous connective-tissue fasciculi, and these in
turn are formed into lobes. By a continuation of the same fasciculi and
fibers which limit a lobe, and in the meshes of which the acini are situated,
a sheath to the excretory duct is formed. Besides the branched, tubular,
mucous glands of the lips, there are occasionally found, at the edges of
the lips, sebaceous glands.

The Glands of the Cheeks (Buccal Glands, Molar Glands). — The
glands of the cheek are also situated in the submucous layer of the mucous
membrane. They, like the labial glands, are of the compound tubular
variety, and when microscopically examined are found to be similar in
structure. They are somewhat larger than the labial glands and pro-
portionately less numerous. The chief duct from each of these glands
usually opens with a narrow mouth on the surface of the oral mucous
membrane, and in its passage through the tunica propria takes a vertical
or oblique direction. In the submucosa the chief duct branches into
two or more smaller ducts, taking up alveoli. As the buccal glands are
somewhat larger than the glands of the lips, they are composed of a greater
number of ducts and alveoli.

The Glands of the Hard and Soft Palate (Palatal Glands).— The
mucous glands of the hard palate are situated in the submucosa and
closely associated with the periosteum. They are compound tubular
glands, and in all essential particulars are similar to the labial and buccal
glands. They are quite numerous (200 to 300), isolated in the anterior
portion, but are grouped into a single row or into two rows posteriorly.
The glands are freely distributed in each lateral half, but are absent at
the median line.

In the soft palate the glands are of the same character, somewhat
variable in size, the largest being found in the uvula. The excretory
ducts from these glands vary in diameter, in the nature of their fibrous
structure, and in the direction taken in passing to the surface. Over
the surface of the soft palate the mouths of these ducts are represented

342

HISTOLOGY.

by minute orifices slightly smaller than the body of the duct, but in the
uvula the opposite condition is present, the mouth of the duct being wider
than the body. The^course taken by the excretory duct is seldom a
direct one, but after receiving all tributary branches passes obliquely
through the tunica propria, and before entering the epithelium turns
at an abrupt angle, and so continues until the surface is reached. The
ducts are lined by a simple columnar epithelium, which in some instances
is ciliated; the walls of the tubes consist of gland-cells and a structure-
less membrana propria. In some instances the surface epithelium may
be reflected for a short distance and partly serve in the capacity of a lining
to the tubular walls.

The Glands of the Tongue (Lingual Glands). — In this organ two
varieties of glands are found, occurring both in the mucous mem-

Fig. 264. — Section through Base of Tongue, showing Serous (ilands.

brane and in the superficial muscular strata, being principally distin-
guished by the nature of their secretions. The gland-cells of the one set
are mucigenous, secreting mucin; these are the. mucous glands. The
other set is productive of a serous fluid, thin, watery, and containing
albumin; these are the serous glands.

The mucous glands of the tongue are found along the lateral margins
and over the root of the organ, being most numerous in the latter situation.

THE SALIVARY GLANDS. 343

They are of the compound tubular variety, and in most particulars are
identical with the mucous glands of other parts of the oral cavity. The
ducts are lined with ciliated columnar epithelium, and the walls of the
duct consist of a homogeneous membrana propria and gland-cells.
The glands occupying the root of the tongue are frequently 'found with
their excretory ducts opening into the follicular crypts. The tubules
consist of a structureless membrana propria and numerous gland-cells,
the latter varying in appearance according to their function or functional
activity. The crypts of the follicles constitute reservoirs for the acinous
glands, and these receptacles frequently extend for some distance beneath
the surface, receiving at various points the main excretory ducts from
the mucous glands. These saccular-like reservoirs are lined by a well-
defined capsule surrounded by a fibrous sheath, internal to which is an
epithelial covering, a prolongation of the common epithelium of the
mouth. Between these two layers are a number of minute, closed lymph-
follicles placed in a single layer. The mucous glands on the lateral walls
of the tongue are, for the most part, situated near the middle or pos-
terior portion. The ducts from these glands usually open directly toward
the cheek, but in rare instances they pass obliquely downward and open
near the proper floor of the mouth. At the tip of the tongue, buried be
neath the mucous membrane and some of the muscular fibers, may be
found a pair of mucous glands (Nuhn's) which open by free orifices on the
under surface. At the root of the tongue, flat, lenticulated elevations of
the mucous membrane are present, beneath which is imbedded con-
globate, glandular substance. These show a central orifice leading to a
small pit lined with tessellated epithelium.

The serous glands of the tongue are compound tubular glands, and are
found in the region of the circumvallate papillae, closely associated with
the taste-buds. The excretory ducts, lined with a simple or stratified
columnar epithelium, the latter sometimes ciliated, open near the base of
the papilla, or between the papilla and its wall. The tubules are similar
to those in the mucous glands, consisting of a delicate, structureless mem-
brana propria and gland-cells. The gland-cells are composed of a frail,
transparent protopasm, containing rounded nuclei.

The Salivary Glands. — The parotid, submaxillary, and sublingual
glands each consists of an excretory duct, branching frequently in a tree-
like manner into smaller ducts, lined throughout with a layer of epithelial
cells. From the smaller ducts terminal branches are given off, which in
turn are lined with epithelium. The other portions of the glands are
invested by columnar epithelium, and arranged like grapes about the

344 HISTOLOGY.

main excretory duct, and consequently belong to the group of racemose
glands. The terminal branches or alveoli attached to the smaller excre-
tory ducts are so numerous that they become much compressed from
pressure, and the grape-like appearance is more or less destroyed, and
but little space is left for interstitial tissue. Each gland is inclosed in a
fibrous connective-tissue capsule, and from this numerous septa of fibrous
trabecular pass to the interior and divide the glandular substance, first into
lobes, these being subdivided into lobules, the lobules by further sub-
division forming the alveoli. The glandular connective tissue is loose
in texture, containing many elastic fibers and lymphoid cells. Fine
bundles of fibrous tissue, together with branched connective-tissue cor-
puscles, constitute the connective-tissue matrix between the alveoli.

The Ducts. — Entering the interior of the gland, the chief duct divides
into a number of large branches, one of which passes to each lobe, each
of these giving off several branches which connect with the several lobules.
Upon close examination the central tube of each lobule is observed to
throw off several small tubes — the intralobular tubes. Following these
are the intermediate tubules, which continue into the terminal compart-
ments. The chief excretory duct consists of a double layer of cylindric
epithelium and fibro-elastic cartilage. Close beneath the epithelium is a
compact membrana propria. The intralobular tubes are each provided
with a distinct lumen. The walls are composed of a membrana propria
lined by a layer of columnar epithelium, the cells of which contain a
central round nucleus.

The Parotid Gland. — The distinguishing histologic feature in this
gland is found in its excretory duct (Stenson's duct), which is provided
with a membrana propria, especially broad and compact, placed imme-
diately beneath the epithelium. The duct is composed of a double
layer of cylindric epithelium and fibrous tissue, intermingled with elastic
fibers. The main duct divides and passes into the intralobular tubes,
beyond which are the intermediate tubules. The intralobular tubes are
lined by columnar cells, while the intermediate tubules are lined by
elongated, spindle-shaped cells. The salivary cells lining the acini are
different in character from those in the submaxillary and sublingual
glands. The parotid gland is a true salivary gland, and the serous
gland-cells composing its epithelial lining are disposed in a single layer.
The cells are columnar or pyramidal in form and composed of a dense
protoplasm, containing a spheric nucleus.

The Submaxillary Gland. — The excretory duct (Wharton's duct),
like the main duct of the parotid gland, is composed of a double layer

NERVE-SUPPLY TO THE SALIVARY GLANDS. 345

of columnar epithelium, external to which is a layer of cellular connective
tissue, the whole being surrounded by a thin stratum of muscular fibers
placed longitudinally. The intralobular tubes are lined by a specialized,
elongated, cylindric epithelium, which, in the intermediate tubules, be-
comes clothed with cubic cells. The acini are lined either with serous
gland-cells similar to those lining the acini of the parotid gland or with
mucous gland-cells, the former being most constant in their presence. The
two kinds of acini are uninterrupted!/ connected. In most instances
there are but a few mucous acini present within the lobule, but occa-
sionally they are found in abundance. The submaxillary is a mixed
or mucosalivary gland.

The Sublingual Gland. — The excretory duct (Rivini's duct) is
similar in structure to the chief excretory duct of the submaxillary gland.
The intralobular tubes are lined with columnar epithelium. The inter-
mediate tubules are not positively known to exist, and it is quite probable
that the intralobular tubes pass directly into the terminal compartments.
The acini are composed of a membrana propria and gland-cells, both
mucous and serous. The former are much more numerous than in the
acini of the submaxillary. The membrana propria is composed of
stellate connective-tissue cells. This gland is also a mixed or muco-
salivary gland.

Blood-vessels and Lymphatics in the Salivary Glands. — The
lobules of the salivary glands are richly supplied with blood-vessels. The
many arterial branches break up into numerous capillaries, which, form-
ing a dense network, surround the acini, being supported by the inter-
alveolar connective tissue. The lymphatic vessels accompanying the
intralobular tubes are in communication with numerous lymph-spaces
which exist between the interalveolar connective tissue and the walls of
the acini. The substance of the gland is further supplied with blood
by numerous plexuses of lymphatics which are carried or supported by
the interlobular connective tissue.

Nerve-supply to the Salivary Glands. — The nerve-fibers distrib-
uted to the salivary glands are both of the medullated and non-medul-
lated variety, and other nerve-tissue in the form of ganglion-cells is
present. The medullated nerve-fibers are abundantly numerous, and
are distributed to all parts of the gland. In many respects the fibers are
peculiarly constructed. They are extremely delicate, made so by the
frail nature of this medullary sheath; they divide and give off so many
branches as to almost give them a feathery fineness. This peculiarity
is especially noticeable toward their extremities, where the fibers lie

346 HISTOLOGY.

between the alveoli and give off minute branches in all directions. The
nerve-fibers are placed in close relation to the tubes and tubules, which
they freely encircle; they perforate the membrana propria and breakup
into finer subdivisions, from which they are distributed to the exterior
of the epithelial cells. In the alveoli two kinds of nerve terminations are
found. The primitive fibers branch between the alveoli and are distrib-
uted to the membrana propria, upon entering which numerous branches
are thrown off which pass to the epithelial cells beneath. The non-
medullated fibers, which are much less numerous, are composed of an
extremely delicate fasciculi of transparent fibers resembling axis-cylinders,
and invested by a sheath of connective-tissue cells containing nuclei.
The distribution of these fibers is similar to the medullated fibers, encircling
the tubes and penetrating the membrana propria, being similarly distrib-
uted to the alveoli.

Literature.

Stohr, "Text-book of Histology."

Strieker, "Human and Comparative Histology," vol. i, 1870.

Klein, "Elements of Histology," 1889.

Sebastian, "Recherches anatomique, physiologiques, pathologiques, les Glans

Labiales," 1842.
Ward, "On Salivary Glands," Tood's "Cyclopedia of Anatomy and Physiology."
Klein and Vernon, Strieker's "Human and Comparative Anatomy," vol. i, chap.

\vi.
Sudduth, "Embryology and Dental Histology," "American System of Dentistry,"

vol. i, part iii.
Brubaker, "Transactions Odontological Society of Pennsylvania," i88o-'a5.
Todd and Bowman, "Physiological Anatomy," vol. i.
Szontagh, " Essays on Minute Anatomy of Hard Palate in Man," 1866.

CHAPTER V.

Muscular Tissues of the Mouth ; of the Lips ; of the Cheeks ; of
the Soft Palate; of the Tongue.

MUSCULAR TISSUES OF THE MOUTH.

Muscular Tissues of the Lips. — The minute bundles forming the
fasciculi of the oral sphincter muscle — the orbicularis oris — are distributed
between the submucosa of the mucomembranous portion and the sub-
cutaneous tissue of the cutaneous portion of the lips. The muscular
fibers radiate in three principal directions upon either side of the median
line: from the angle of the mouth toward the median line, and from the
fleshy slips of the maxilla and mandible — the musculi incisivi. As the
fibers from the angle of the mouth pass to the substance of the lip, they
are arranged in a laminated manner. ' When the median line is reached,
one set of fibers terminates somewhat abruptly in the subcutaneous tissue,
another set is continued beyond the median line and attached to the cutis
of the opposite side, while a third set, without crossing the median line,
is attached to the incisive fossa? of the maxilla and mandible. The numer-
ous muscular fibers of the internal labial or mucomembranous portion,
and the external, facial, or cuticular portion, penetrate the parts and
terminate in close proximity to the epithelium or to the base of the papillae.
Delicate, hair-like fibers which are continuous with the sarcolemma
slightly penetrate the cutis and membrana propria. A few of the fibers,
which may be classed with the terminals of the outrunning muscles
from the lips, are arranged in a number of fasciculi in the subcutaneous
portion, pass through the fasciculi of the orbicularis oris, reach the sub-
mucous tissue, where they cross and recross one another, and finally pass
into the membrana propria, where they end in fan-like terminals. The
fasciculi of the orbicularis differ somewhat in the upper and lower lips;
in the former the bundles are strongly developed toward the angle of
the mouth, while in the latter the median bundles are the strongest.
The labial muscular tissues are of the transversely striated variety. The
fibers are cylindric in form, having rounded or pointed extremities in
the interior, and broad or flattened ends where they come in contact
with the periosteum. When examined with a high power each fiber

347

348 HISTOLOGY.

shows alternately broad and narrow striae, the former being dim, while
the latter is bright in appearance. With a stronger power both the broad
and narrow striae are seen to be transversely striated.

Muscular Tissues of the Cheeks. — The muscles entering into the
construction of the lateral walls of the mouth have already been described
in part I, page 12, giving the relations existing between the individual
muscles, together with the general disposition of the various fasciculi.
Histologically considered, these muscles partake of all the characteristics
of striated or voluntary muscular tissue. In the body of the buccinator
and masseter muscles the fibers are cylindric and have definitely pointed
or rounded ends. Near their termini, particularly in the latter muscle,
the inner extremities of the terminal fibers are pointed, while the outer
ends, or those by which the attachment is formed, are broad and rather
flat.

Muscular Tissues of the Soft Palate. — The disposition of the
striated muscular tissue of the soft palate is extremely complicated. The
azygos uvula, the only true longitudinal muscle in the soft palate, has
its origin from aponeurosis of the soft palate and from the nasal spine of
the palate-bone, the fibers passing backward upon either side of the
median line. This is a double muscle, and near its point of origin the
two portions are distinct and separated by a definite space, but upon
reaching the base of the uvula they become closely associated. The
fasciculi do not continue to the apex of the uvula, but immediately beyond
the center of its length are thrown out fan-like toward the sides, terminat-
ing in a manner similar to the fibers of the lips. In passing from before
backward a number of small fasciculi are given off, which reach out
laterally and traverse the glandular lobes, completely surrounding them,
after which they again return to the principal fibers at the median line.
The palatopharyngeus muscle is divisible into two parts, the upper
extremities of which lie partly in front and partly behind the levator
muscles. The greater number of the fibers of one set, situated in front
of the levators, form a curved, flattened aponeurosis. The fibrous
border of the hard palate serves as an attachment for the convex border
of this portion, while the other border, which is concave, is directed
toward the arch of the levators. The fibers of the palatopharyngeus,
situated behind the levators, form a number of loose fasciculi inter-
spersed by fat-cells. In passing toward the free border of the soft palate
the fibers become much more delicate, and, separating, some course in
front and others behind this muscle. In this location the fibers become
closely associated with the glands, and either end here or are continued

MUSCULAR TISSUES OF THE TONGUE. 349

to the submucosa, or even to the membrana propria of the mucous
membrane. The fibers of the palatopharyngeus unite with the fibers of
the levators, and an arch-like fasciculus is formed by this union which,
subdividing, passes in front of the azygos uvulae to the opposite side.
All of these fibers run outward and downward, and unite with the
extremities of the other palatal muscles, the fibers of which are some-
what more regularly distributed. Like the muscles of the lips and
cheeks, the several fasciculi of the palatal muscles form a delicate
plexus, and a quantity of fatty tissue is found between the various
fasciculi.

Muscular Tissues of the Tongue. — The tongue is divided into
two equal lateral portions by a median septum — the septum linguce.
This central septum, composed of a vertical layer of compact, fibrous,
connective tissue, extends the entire length and depth of the lingual
median line. Beginning at the hyoid bone, it gradually increases in
prominence until the middle of the organ is reached, beyond which point
it becomes less pronounced and finally disappears near the tip. The
bundles of the muscular tissues are arranged longitudinally, transversely,
and vertically. The former lie immediately beneath the mucous mem-
brane, including the superior lingualis above and the inferior lingualis
below, together with the greater part of the styloglossus. The superior
lingualis extends from the base to the tip of the organ, and by short
fasciculi its fibers are attached to the overlying tissues. The fibers of
this muscle are placed between the hyo- and styloglossi muscles of the
opposite side, both of which overlap the fibers of the lingualis near the
base of the tongue. The inferior lingualis also gives off several small
fasciculi and fibers to the mucous membrane beneath, and is composed
of two bands which reach from the base to the apex, each being
placed between the hyoglossus and genio-hyoglossus muscles. The
transverse fibers, which are placed between the superior and inferior
lingualis muscles, originate from the septum linguae, and form the bulk
of the organ. From their point of origin these fibers course outward and
upward to the sides of the tongue. Those fibers which are vertically
disposed decussate with the transverse fibers, and pass from the dorsum
toward the under surface of the tongue, the fibers curving gracefully with
their concavity directed toward the under surface. In most instances
the ascending vertical fibers, as well as the transverse fasciculi, pass
between those longitudinally disposed and connect with the submucosa.

CHAPTER VI.

Tissues of the Teeth — Enamel; Dentin; Cementum; the Tooth-
Pulp. — The Alveolodental Membrane.

TISSUES OF THE TEETH.

Enamel. — The enamel, which forms a cap-like covering of varying
thickness over the entire crown of the tooth, is a vitreous, hvaline sub-

FlG. 2J5. — Section of Enamel from Human Tooth (Specimen by J Howard Mummery.)

X 350. {After Williams.)

stance, containing but little, if any, organic matter. The thickness of
the enamel cap appears to be strongly influenced by the function of the

35°

ENAMEL. 351

different parts of the tooth-crown, being thickest over the cutting-edges
of the anterior teeth, while in the cuspidate teeth, the entire occlusal
surface is provided with the thickest enamel layer. It is about evenly
distributed over the lateral walls of the crown, but as the cervical line is
approached, its thickness is gradually diminished (Fig. 226). Chemically,
enamel is composed of the salts of lime, calcium phosphate predominat-
ing. Calcium carbonate, magnesium phosphate, and calcium fiuorid
are present in smaller quantities. The proportionate quantity of lime-
salts in enamel is not fixed, a slight variation in density occurring in the
enamel of different individuals. These essential differences are regulated
by the proportionate quantity of calcium phosphate and carbonate — a
greater amount of the former being productive of additional hardness,
while an increase in the latter beyond the minimum amount decreases
this quality. As a general rule, the teeth of males contain a greater
amount of calcium phosphate than the teeth of females, as shown by the
following analysis by von Bibra:

Calcium phosphate and fluorid, 89

Calcium carbonate, 4

Magnesium phosphate, 1

Other salts,

Cartilage, 3

Fat

AN.

Woman.

.82

81.63

•37

8.88

•34

3-55

.88

•97

■39

5 97

.20

a trace

■59

5-97

Total organic, 3 .

Total inorganic, 96.41 94 .03

In general structure enamel is composed of numerous hexagonal
prisms, with a common direction at right angles to the long axis of the
tooth. These prisms are known as enamel prisms, enamel fibers, or
enamel rods. While the general direction of the fibers is, as previously
stated, nearly at right angles to the body of the tooth, they do not pursue
a perfectly straight course in passing from the dentin to the surface, but
are disposed in a tortuous or wave-like manner. The enamel prisms
may be said to sit on end against the surface of the dentin, minute depres-
sions in the latter receiving the extremities of the rods. The direction
of the enamel prisms, as compared to the body of the tooth-crown, varies
according to the part of the crown which they occupy. Taking the
entire crown of the tooth, they radiate in such a manner from the surface
of the dentin that at the cutting-edge or occlusal surface of the tooth they
are more or less vertical, while over the lateral surfaces they tend to the
horizontal direction. An examination of the prisms whemisolated and

352 HISTOLOGY.

decalcified exhibits numerous evenly distributed varicosities, producing
a transversely striated appearance to the rods. Tomes has pointed out
that the enamel rods are variously disposed in that portion of the enamel
most closely associated with the dentin. On the cusps of the teeth they
are twisted and curved in various directions, while near the surface on the
incisors they are uniform and straight. In general, the enamel rods,
which begin on the surface of the dentin, are continuous through the
entire thickness of the enamel. In passing from the interior to the exte-
rior, the individual rod, to occupy a proportionate space in all parts,

Enamel

Fig. 266. — Enamel and Dentin from Human Tooth, showing Gradual Reduction in the
Thickness of the Former as the Cervix is Approached. X 20.

would have to increase in diameter; but this it does not do. In conse-
quence of this arrangement there exist numerous supplemental or per-
ipheral rods, which extend but a short distance from the surface, filling in
the interprismatic spaces formed by the longer rods. With the exception
of the faint transverse striations, the enamel prisms appear to be structure-
less. A variety of opinions have been expressed in regard to the cause
for the striated appearance of the enamel rods. It was claimed by Hertz
to be attributable to a temporary arrest of calcification, but more recent
investigation has shown the cause to be the presence of varicosities in the
individual fibers, precisely as the varicosities in the muscular fibers pro-

ENAMEL.

353

duce the striated appearance in that tissue. Bodecker asserts that fully
developed normal enamel is non-striated, and Von Ebner practically
makes the same statement, claiming that they are due to the preparation
of the specimen, which usually being mounted in Canada balsam, suffers
sufficiently from the slight acid reaction to produce the striated appear-
ance. These statements Williams emphatically denies, saying that, while
in some specimens the varicosities are apparent in some parts, they are
decided in others. If this be accepted, it would seem to entirely over-
throw the theory of Von Ebner in regard to the action of the acid, which
would be distributed to all parts alike.

Dentin.

Enamel.

Fig 267. — Comparison in the Appearance of the Enamel and Dentin under Low Power

of the Microscope. X 40.

^According to Williams, the varicosities of one enamel prism are
opposite those of the adjoining prisms, and by the coming together of
the varicosities the prisms become united by means of processes which
they send out. In like manner the varicosities upon the same rod are
connected by processes running parallel with the prism. According to
Von Ebner, enamel is traversed by numerous minute canals, and Heitz-
mann claims to have found organic fibers in its substance. Williams,
while admitting the enamel structure to be far more complex than past
research has shown, appears to have fully demonstrated that neither canals
nor organic fibers are present — in fact, he denies the presence of the least
23

354

HISTOLOGY.

trace of organic matter in this structure. The interprismatic matrix,
heretofore considered by most authorities to be an organic structure,
now appears, by the thorough methods employed by the last-named
gentleman, as a transparent, inorganic substance. By numerous experi-
ments he was enabled to secure a specimen in which the interprismatic
spaces of one layer were not backed up by the rods of another layer.
In some instances the specimen showed the rods well separated and the
interspace closed by a perfectly transparent substance, in the interior of
which might be seen connecting processes passing from one rod to another.

Enamel
Rods Fully
in Trans-
verse Sec-
tion

Enamel
Rods vari-
ously Dis-
tributed

Enamel
Rods Not
Fully in
Trans-
verse
Section

Enamel
Rods of
Irregular
Form

Fig. 268. — Human Enamel Transverse Ground Sections. (After Cy. si.)

That the enamel is practically of inorganic material, and therefore not
capable of transmitting or receiving sensations, may be demonstrated by
the simple experiment of immersing a thin section of this tissue in a weak
solution of chromic acid, the result of which will be a speedy separation
of the enamel prisms which have been liberated by the destruction of the
interprismatic substance. What does this signify? Chromic acid is
one of the best preservatives of organic tissue known, and if the cement-
ing substances in enamel were organic or even partly so, the prisms by
this test would not be freed, but instead would become more firmly ce-
mented together. Therefore we infer from this that the interprismatic

ENAMEL. 355

substance is even less highly organic than the prisms themselves, these
not being acted upon until after the material which holds them together.
In addition to the striated appearance formed by the varicosities of the
individual prisms of fully developed enamel, other structures of a different
character, and upon a much larger scale, are present and known as the
"brown stria oj Retzius" (Fig. 269). These markings, readily seen
with a low power, are of a brownish color, and run nearly parallel with
the surface of the dentin or enamel. Those striae nearest the surface of
the dentin are inclined to follow the contour of that structure, extending

Fig. 269. — Thick Section of Enamel of Human Tooth, showing Brown Striae of Retzius.

X 40.

in many instances the entire length of the crown. The lines nearest the
surface are longest in the region of the cutting-edge, or occlusal surface
of the crown, becoming shorter as the neck of the tooth is approached,
being directed at an acute angle to the surface of the dentin at that point.
A number of theories are advanced to account for the presence of the
"brown striae of Retzius." Tomes suggests that, coinciding as they do
with the outer surface of what was at one time the primitive enamel cap,
they might be considered as in a measure outlining the stratifications of
the primary deposit. Another theory, but one seemingly without founda-
tion, is to the effect that the striae are produced by an arrest in the calci-

356

HISTOLOGY.

— W— ■—&&? :Vf '"J3r3

iLl^

3rown striae
of Retzius.

Lines of f
Schreger. (

■ — '** i£&-* - K J -v? J

Enamel.

Dentin.

Fig. 270. — Enamel and Dentin, Human Tooth. (After Gysi )

Oral Epithelium
Heaped up
over Band

Band

Connective Tissue

FlG. 271. — Vertical Section through Jaw of Human Embryo. Formation of Tooth-band,

about Sixtieth Day. X 300.

ENAMEL.

357

fying process; while a third theory attributes the cause to a variation in
the character of the nourishment taken by the mother during pregnancy.
The acceptance of this latter theory would seem to indicate a set diet
for all mothers and at stated intervals, the striae always being present
and somewhat regularly distributed throughout the tissue.

Still another set of lines or markings are to be observed in the sub-
stance of sections of enamel, these being known as the "lines of Schreger."
Figure 270 shows these lines as they appear in the enamel by reflected
light, the same being quite invisible by transmitted light. The presence

Epithelium

.'.;.- (■■■

Band

(Older 1

I Layer
Infant
Layer

Connec-
tive
Tissue

Fig. 272. — Vertical Section, Tooth-band, Human Embryo. Tenth Week. X 300.

of these lines is due to the various directions assumed by the contiguous
groups of enamel rods. Beginning at the surface of the dentin they are well
defined, but gradually become less marked as the exterior of the enamel
is approached. At the line of union between the enamel and dentin ir-
regularly formed cavities are occasionally observed, into which the denti-
nal tubules may extend, and in rare instances individual tubules may
pass beyond the boundary-line of the dentin and enter the enamel, but in
all probability both of these conditions are pathologic. Such a state of
affairs could hardly be considered normal when we take into considera-
tion that the dentin and enamel calcify in opposite directions, and that the
outer wall of the former is completed before enamel calcification begins.

358 HISTOLOGY.

Development of Enamel. — Preparations for the development of the
enamel begin toward the close of the second fetal month, appearing first
as a multiplication of the primitive epithelial cells in the form of a con-
tinuous linear projection extending somewhat obliquely into the sub-
jacent connective tissue. From this crest, or tooth-band, the germs for
the future enamel organs are given off. These primary dental bulbs,
as they are called, number one for each tooth to be generated, and coinci-
dently with their appearance an aggregation of closely associated connec-
tive-tissue cells make their appearance in the surrounding submucous

Dental Ridge

$m A

Band w-»**fcy33l

Epithelium

Location of Future
Enamel Organ

******&•

.:>>.•

Connective Tissue

Fig. 273. — Same as Figure 272. About Twelfth Week. X 200.

tissue. This papilla-like specialization of the submucous tissue is the
primitive dentin germ, or dentin papilla. It will thus be observed that the
enamel is a product of the surface epithelium, ectodermic, while the dentin
is generated from the connective tissues, mesodermic. Soon after the ap-
pearance of this club-shaped thickening, or tooth-bulb, by further differ-
entiation its form becomes bell-shaped, with the concavity directed toward
the surface. The dentin papilla gradually pushes into the concavity
of the forming enamel organ, and at a later period the odontoblastic cells
are generated about the periphery of the papilla, closely followed by a
surface calcification of the dentin. Soon after the forming of the external
layers of dentin the ameloblasts or enamel-forming cells become active,

ENAMEL.

359

and a deposition of enamel prisms takes place upon the exterior of the
dentin cap.

Before taking up the subject of enamel calcification, brief reference
will be made to the further development of the enamel organ. As the
growth of this organ proceeds there is, as the result of a rapid proliferation
of the cellular structure, a marked tendency for the organ to become
separated from the tooth-band. The peripheral cells or external epithe-
lium are columnar or prismatic, and remain so, while those in the center,
primarily polygonal, soon become transformed into a radiating network

Surface of Jaw

Epithelium
Upper Jav

Epithelium X

■> ,'i

"• - ■ c? ' f.-y :

Neck of Cord

■ Stellate Retic-
g ulum of

Enamel Organ

I Enamel Organ
I

Dentin Papilla

Forming Bone

Fig. 274. — Vertical Section through Tooth-band, Cord, Dentin Papilla, and Enamel Organ,

about Fifteenth Week. X 150.

or stellate reticulum. The appearance of a stellate reticulum is first
observed to take place in the cells occupying the central portion of the
enamel-organ, this cellular transformation progressing from the center
outward, but ceasing before reaching the columnar surface cells contigu-
ous to the dentin papilla. Between the enamel cells and the stellate
reticulum is a layer of unaltered cells — the stratum intermedium. In
the earlier stages of the development of the enamel organ the peripheral
cells are alike, being columnar or prismatic, but almost coincident with
the appearance of the dentin papilla, the cells most closely related to it
are observed to become elongated, and form the internal epithelium of the
organ. As the cells forming this internal epithelium become elongated,

3 6 °

HISTOLOGY.

their nuclei, instead of occupying the center of the protoplasmic body,
are carried to their extremities. It will thus be seen that the completed
enamel organ consists of four divisions or layers of cells. Beginning
with its convex surface is an external epithelium or outer tunic, succes-
sively followed, in passing toward the dentin papilla, by a stellate reticu-
lum, stratum intermedium, and an internal epithelium or inner tunic.
As the growth of the enamel organ proceeds, the tooth-band becomes

FlG. 275. — Section of a Developing Tooth of Lamb.
a, Ameloblasts; D, dentin; P, dental pulp.

X 600.

smaller and smaller in size, until finally a complete rupture takes place.
This rupture, however, does not occur until the enamel organ has about
or fully completed its development, and, after remaining so long under
the influence of the oral epithelium, it must be considered, as before stated,
an epithelial structure. It is through the agency of the internal epithelial
cells of the enamel organ, the enamel cells or ameloblasts, that calcification
of the enamel take? place, and that subject will next be considered.

ENAMEL.

;6i

Amelification (Fig. 275). — -Two theories are advanced in regard to
the calcification of the enamel. In one it is claimed that the ameloblasts
or enamel cells became directly calcified or converted into enamel; in the
other the ameloblasts are simply considered as controlling agents, by
secreting or depositing the calcium salts which form the enamel prisms.
In the latter theory it is generally believed that the enamel is secreted or
shed out from the extremities of the ameloblasts, thus being productive

Dentin

ffim

Enamel
I Rods

ft Outer

£— Layer of
Enamel

WMMkiW^

JlhI MrJt '

Amelo-

uNC **9E<SKjE ** /

blasts

Section of
Papillae

Fig. 276. — Section of Incisor of Rat. X 200. (After Williams.)

of enamel fibers corresponding in size and position to the secreting cells.
By the direct calcification of the ameloblasts, it would be natural to
expect the process to begin on the exterior of the cell and gradually pass
into its interior, the central portion being the last to calcify. This
would result in an enamel prism corresponding in size and form to the
generating cell, and in a measure this similarity between the calcified
and uncalcified structure does exist and is one of the potent factors

362 HISTOLOGY.

in the recognition of this theory, but it is hardly sufficiently convincing
to warrant a general acceptance of the belief.

By examining figure 275 the importance of the secretory theory is
favored. Here we note that the early formed enamel at A records what
appears to be the definite action of the ameloblasts by prolongations of
partly calcified tissue extending from the cells, these markings corre-
sponding in number and location to the cells themselves. Between these
prolongations and the ameloblasts are many highly refractive granular
bodies which seem to be the actual lime deposit shed out from the free
extremities of the cells, from which they pass into the substance of the
then organic matrix beyond at A and form the enamel prisms. Williams
takes exception to this latter view, substantiating his opinion by stating
that while the ameloblasts of many animals are similar in shape and
arrangement, the enamel produced from these similarly arranged cells
varies greatly in structure. The same writer also states that, when such
a similarity of arrangement exists between the ameloblasts and the enamel
prisms, it occurs near the commencement of enamel calcification, and
that at a later period the relative position of the ameloblasts and the
prisms is always in longitudinal section. Dr. Williams calls attention
to the fact of the enamel prisms or rods not extending through the entire
distance between the enamel cells and the calcified dentin (Fig. 275).
That part of the structure lying between the ameloblasts and the extrem-
ities of the enamel rods is made up of a double set of fibers, some of
which are almost at right angles with long axis of the ameloblasts. In
figure 277 D the two sets of fibers previously mentioned are found to
join and become closely interwoven.

The ameloblasts are connected with the cells of the stratum inter-
medium, and more recent investigation goes to prove that this latter
structure is directly interested in furnishing to the ameloblasts the proper
material for the calcifying process. The stratum intermedium cannot,
however, take part in the primary enamel calcification, as this process
commences before the stratum intermedium is fully developed. This
being the case, it is generally supposed that the stellate reticulum furnishes
the material for the upbuilding of the first enamel prisms.

Lying between the free extremities of the ameloblasts and the enamel
in the course of formation is what has been generally considered as a
structureless basement membrane or membrane praformativa. The
existence, exact location and structure of this membrane has been and
still remains a matter of conjecture. The generally accepted theory
appears to be that given above, but Williams refers to it as a layer of

ENAMEL.

;63

newly formed enamel, and does not consider it as a structureless mem-
brane. The structure is to be observed in figure 277, at either extremity
of the ameloblastic cells, this writer claiming that the so-called structure-
less membrane is present at both of these points.

As to the formation of the enamel rods, Dr. Andrews, in 1894,
referring to the presence of calcoglobulin in the enamel cells considered

Outer
Fibrous
Enamel

Layer

Ameloblasts

Position of Capillary Loop

Fig.

277.

-Section of Incisor of Rat, showing Partial Decalcification of Enamel.

{After Williams.)

X 600.

these refractive bodies as calcospherites, which, after being taken up by
the ameloblasts, were excreted by them, and after coalescing formed
globules of larger size, from which the rods were built up. Williams
partly agrees with this statement, but he is of the opinion that the calco-
spherites coalesce while in the ameloblasts, forming large, spheric bodies,
but that the deposit of this substance is in no way productive of building

3 6 4

HISTOLOGY.

the enamel rods. The theory of Tomes in regard to the forming of
enamel rods was that the walls of the ameloblasts themselves became
calcified, while the contents of the cells also became solidified, the first
forming the interprismatic substance, while the second became the
enamel rod. Whatever theory be accepted as to the formation of the

Fig. 278. — Section of Developing Tooth of Embryo Lamb. X 150. (After Williams.)

a, Forming Dentin; 6, Forming Enamel; c, Stratum Intermedium; d, Inner Ameloblastic
Membrane; e, Outer Ameloblastic Membrane; /, Ameloblasts.

enamel rods or prisms, there appears to be no question in regard to the
general process of enamel calcification. In the first place, an organic
matrix is formed, into which the first-formed layer of enamel is deposited.
Gradually the organic matter disappears, leaving behind the inorganic
elements closely resembling in appearance the organic matrix, which
it has by atomic change supplanted. The question of an organic inter-

ENAMEL.

365

prismatic cement-substance is also one upon which various writers dis-
agree. Klein partly believes that such a substance does exist, basing
his opinion upon the fact that the ameloblastic cells, in common with all
epithelial cells, are separated from one another by a homogeneous inter-
cellular substance, and that a certain proportion of this organic substance
must remain between the enamel prisms after calcification. Dr. Sudduth,
by a series of experiments made some years ago, appeared at that time

Fig. 279.

to have furnished conclusive proof that an organic, interprismatic cement-
substance does not exist between the enamel prisms of fully developed
enamel. By the use of a dilute solution of chromic acid, the action
of which is the preservation of organic substance, the prisms were liberated,
which would not have been the result had they been cemented by an
organic cement-substance. By substituting dilute muriatic acid, the
action of which is the destruction of organized tissue, the prisms were not

366 HISTOLOGY.

liberated, the acid acting evenly upon the whole mass of enamel, and
finally resulting in its complete destruction, not leaving the slightest trace
of an organic matrix behind. Dr. Williams claims that this transparent
cement-substance is formed by the distribution of a translucent liquid
substance about the previously formed pattern for the enamel rods.
This pattern, generated through the activity of the enamel cells, is com-
posed of a translucent material somewhat more solid than that substance
which surrounds it. These two substances calcify together, the latter
forming the enamel prisms, while the former creates the cement or inter-
prismatic substance. There is no better method by which to study the
character or mode of development of a growing or matured tissue than
by an artificial disassociation of its component parts. In enamel it
matters but little in what part of the tissue or at what period of its growth
the examination be made to learn of the action of the decalcifying agent;
it will never be found to take place in a manner corresponding to the
direction of the enamel fibers, but decalcification takes place in older
enamel more in the form of a general breaking up of the structure, as
shown at A in figure 279, while at B, which represents the newly formed
tissue, the action is one which appears to indicate a breaking up of the
interprismatic substance, favoring the theory of secretory amelification.

This distinction between the enamel cells and their product possesses
none of the characteristics to properly classify it as structureless. There
is but little doubt as to its character, being the primary product of the
ameloblasts, while the corresponding zone at the distal end of the
enamel-forming cells results from the functional activity of the stratum
intermedium.

Figure 288 shows a section near the point of the cusp of a developing
molar and exhibits a portion of the enamel organ at a time immediately
prior to the beginning of enamel calcification. A is the uncalcified dentin,
B the ameloblasts, now closely associated, and a very regular layer of
elongated cells, and behind these another layer of cells, which undoubt-
edly serve as feeders to the ameloblasts, the stratum intermedium. This
section is especially valuable in that it shows a number of capillaries
distributed through the body of the stellate reticulum and actually pene-
trating the stratum intermedium, as seen at B.

When viewed with a low power, these minute blood-vessels appear
to form a complete network, and in the district between the cusps pervade
the entire structure, from the stratum intermedium on one side to the
same cells on the other. The appearance of this animated vascular sup-
ply to the enamel is coincident with the process of calcification, for during

ENAMEL.

367

the early life of the tooth-germ it is non-vascular. The growth of enamel,
strata upon strata, from within outward is therefore by the direct cal-
cification of the enamel cells or ameloblasts, and while this is going on, and
as long as the crown of the tooth is incased in its epithelial cap, the enamel
organ, the growth of the tissue is stimulated through the blood-vessels

Fig. 280.

everywhere present in the stellate reticulum. The presence of this special-
ized blood supply to the central portion of the enamel organ was for a long
time doubted, but at present it can be readily observed (Fig. 279). As
soon, however, as the tooth passes through the surface tissue, carrying
with it the external epithelium of the enamel organ as the enamel cuticle,
the possibility of nourishment has been cut off, and after a little time it

368 HISTOLOGY.

becomes a petrified dental epithelium, no longer nourished and absolutely
non-vital.

Dentin.— This tissue, which constitutes the principal bulk of the
hard part of the tooth, forms a complete cap-like investment over the
pulp, from which it is generated. It is white or slightly yellowish-white
in color, somewhat elastic, and a trifle harder than bone, which it resem-
bles in many of its characteristics. In a perfectly developed tooth no part
of the dentin appears upon the surface, that part within the crown being
covered by the enamel, while that of the root is inclosed by the cementum.

■ Dentin
Enamel

Fig. 281. — Section through Crown of Human Cuspid. X 30.

While the thickness of the dentin varies somewhat over the different parts
of the tooth, there is a decided disposition to an equal distribution in
every direction. Dentin, unlike enamel, consists of an organic matrix —
a reticular tissue of fine fibrils richly impregnated with the salts of cal-
cium, in this resembling the matrix of bone. Traversing the matrix
are long, fine canals or tubes (Fig. 289) — the dentinal tubules — which
pass from the margins of the pulp toward the surface. Immediately
surrounding the dentinal tubules the matrix is especially dense, forming
a lining or sheath to the tubes, known as the dentinal sheaths. Occupying
the lumen of the dentinal tubules are solid elastic fibers — the dentinal
fibers. Dentin, therefore, presents for examination, first, the matrix;

DENTIN. 369

second, the dentinal tubules; third, the dentinal sheaths; and fourth,
the dentinal fibers.

The Matrix. — As previously stated, the matrix is composed of
organic and inorganic substances, but the proportionate quantity of
organic and inorganic constituents is so variable that it is impossible
to furnish a definite chemic analysis. The relative quantity of organic
and inorganic matter is not only variable in the teeth of different individ-
uals, but is continually changing in the teeth of the same individual, the
former being present in larger quantities during youth and gradually

Fig. 282. — Section through Root of Human Incisor, showing many Dentin Tubules in

Transverse Section. X 200.

diminishing as age advances. From an examination of perfectly dried
dentin, the following approximate analysis has been obtained:

Organic matter (tooth-cartilage), 27.61

Fat, 0.40

Calcium phosphate and fluorid, 66.72

Calcium carbonate, 3-36

Magnesium phosphate, 1.08

Other salts, 0.83

The organic basis of the matrix appears to be structureless and
transparent, and, although closely resembling the matrix of bone, is
not identical with it. While the matrix is usually structureless, there
24

37°

HISTOLOGY.

are instances in which the presence at one time of connective-tissue fibers
is indicated.

The Dentinal Tubules (Figs. 283, 284, 285). — Beginning by a free
opening about the walls of the pulp-cavity, the dentinal tubules permeate
the matrix in all directions. The tubules are generally disposed in a
direction, perpendicular to the surface, so that in different parts of the
tooth they radiate in various directions. Beginning upon the surface
of the pulp-cavity, at which point they are of greatest diameter, they
pass more or less in a spiral manner toward the surface (Fig. 285), before
reaching which they become gradually reduced in size, as a result of the

Lacuna . ■*. _"--'V

Dentinal Tubules

£ Cementum

Dentin

Fig. 283. — Dentin and Cementum from Root of Human Molar. (After Gysi.)

numerous branches which they give off (Fig. 284). The branches given
off from the main tubes are quite variable in size, and anastomose with
one another or with the branches from other tubules. In the region of
the pulp the tubules are so closely associated that but little space is pro-
vided for the intertubular substance or matrix; but as the surface is
approached they become more widely separated, and, in consequence,
the matrix substance is present in greater abundance. While the general
direction of the tubes is perpendicular, they do not pursue a direct course,
but are more or less curved as they pass from within outward. The
curvature of the tubuli may be divided into two classes — long curves
and short curves — usually referred to as the primary and secondary curva-
tures of the dentinal tubules. The primary curvatures are few in number
and are most prominent in the crown, while the secondary curvatures, prin-

DENTIN. 371

cipally found in the roots, are smaller and more numerous. The branches
from the main tubes terminate in various ways, either by anastomosis,
by gradually fading out into hair-like terminals, or by ending in hooks
and loops. In rare instances they are said to enter the substance of
the enamel or cementum, but it is doubtful if they do so normally. The
branches from a main tube are usually two in number, the latter being
almost equal in diameter to the former, and from this first set of branches
a number of minute branches are given off almost at right angles. In
the crown this latter class of tubules are seldom observed, excepting
near the enamel margin, but in the root they are everywhere noticed.

Fig. 284. — Longitudinal Section through Root of Human Molar. Branching of the

Dentinal Tubules. X 200.

Small varicosities are frequently present, but not in sufficient numbers
to produce a striated appearance on the surface of the dentin.

The Dentinal Sheaths. — While the dentinal tubules ramify through
the matrix in the form of well-defined channels, the walls of the channels
are not formed by the matrix, but by an indestructible substance the
exact character of which is not fully understood. The walls of the tubes,
or the dentinal sheaths, as they are termed, are believed by some histolo-
gists to be calcified, while others, though acknowledging their apparent
indestructibility, are doubtful as to the correctness of this theory. Neu-
mann being the first to accurately describe the walls of the tubules, they
have become known as "Neumann's sheaths." The existence of the

372

HISTOLOGY.

dentinal sheaths may best be demonstrated by subjecting the tissue to
the action of strong acid for a sufficient time to destroy the intervening
matrix, which process usually requires several days. The fibrous mass
remaining will be found to contain a collection of tubes, which, however, by
careful examination, are found not to be the dentinal tubules themselves,
but the walls of these canals. Magitot and Sudduth deny the existence of a
wall to the dentin tubes. Tomes, while inclined to the belief that the tubes
are provided with definite walls, suggests that they may have been pro-
duced artificially during the preparation of the specimen, and that they
are only brought into existence by the action of the agent used for this

Fig. 285. — Transverse Section through Root of Human Molar, showing the Curvature of
the Dentin Tubules about the Pulp-canal. X 40.

purpose. In conclusion, the same writer adds that that part of the
matrix immediately surrounding the fibril differs in its chemic con-
stituents from the body of the matrix.

The Dentinal Fibers. — Occupying the lumen of each dentin tube is
a soft, elastic fiber, which is continuous with and has its origin from the
odontoblastic cells upon the periphery of the pulp. The existence of
these elongated processes of the odontoblasts having first been demon-
strated by Tomes, they are otherwise known as Tomes' fibers. By
means of these fibers, which not only fill the lumen of the larger tubes,
but the minute branches as well, the substance of the dentin is both

DENTIN. 373

nourished and rendered slightly sensitive. There is still some doubt as
to the real nature of the fibrils, but, if they are processes from the odonto-
blasts, it would appear that the substance would be identical with that
of the cell-protoplasm. Bodecker claims that they are not round but
inclined to angularity, but Tomes infers that this form has been produced
by the action of some reagent. Klein advances the theory that the odon-
toblasts are active in the generation of the matrix for the dentin only,
and that the dentinal fibrils are not processes from them, but originate
from cells intervening between the odontoblasts and connecting with the
dentin tubes. It has never been fully demonstrated that true nerve-

'"''

Fig. 286. — Longitudinal Section through Root of Human Tooth, showing Primary
Curvature of Dentin Tubules. X 40.

fibers enter the dentin along with or in the substance of the dentinal
fibril, but, while the evidence is not at present forthcoming, there is but
little doubt that the sensitiveness of the dentin is produced by the presence
of organized tissue in the tubuli. Some contend that the contents of the
tubules are made up of, first, a creative portion, that given off directly from
the odontoblasts; second, a circulatory portion, a minute vessel travers-
ing each tubule, entering either by the side of the cell-bodies or passing
through them, and that the nerve terminals are distributed in the same
manner. Others say that minute nerve-filaments from the pulp pass
directly through the odontoblasts and are continued in the center of the
tubule surrounded by a simple connective tissue, the cell process, and

374 HISTOLOGY.

that in this way sensations are conveyed. It is now generally conceded
that dentin is a highly organized connective tissue; that it has a circula-
tory system and is endowed with sensation to a slight degree; that these
conditions are brought about not by actual entrance into the tubules of
separate vessels and nerve-filaments, but more in the way of the tubules
being occupied by a general connective-tissue substance resembling in
all essential features the pulp itself, being the semi-fluid interfibrillar
ground-substance of the pulp; that dendrites of sensory neurons every-
where present in the pulp, after losing their medullary sheaths divide
into fine varicose fibers and become closely associated with the peripheral

Cemen-

tum

1 JWM,

Dent

)f . Pulp-canal

Fig. 287. — Transverse Section through the Root of a Human Incisor, showing the Dentin
Surrounded by the Cementum. X 30.

cells, pass between these, and enter the cone-shaped openings of the
tubules and terminate soon after doing so.

While the microscope reveals in some instances what appear to be
prolongations from the dentinal fibers penetrating the enamel, or between
its prisms, such a condition is improbable if not impossible. If this
arrangement is present at all, it is so slight as to have no influence what-
ever over the enamel either as to nourishment or sensation. No conclu-
sions can be drawn with positive certainty from sections, since the slightest
deviation from parallelism in the surfaces may easily produce deceptive
appearances. It is just as common, and even more so, to find hair-like
lines interwoven and running parallel with the surface of the dentin

DENTIN.

375

immediately between this tissue and the enamel, as it is to see slight
fibers crossing beyond their boundary-line to penetrate the enamel.
The most likely place of all to find such a condition would be in the
beginning of calcification, and here it is never observed. The peripheral
pulp-cells, usually all classed as odontoblasts, are never found outside
their own territory, the dentinal papilla; but their location in the beginning
on the very surface of the papilla, almost in direct contact with the inner
tunic of the enamel organ, would make it possible for their processes,
when appearing, to penetrate between the cells of the enamel organ if
they were grown out from the body of the cells from which they spring.

Ceraentum \

Dentin

Granular
Layer

Fig 288. — Tomes' Granular Layer. X 40.

This, however, they do not do. They do not grow out from the cell-
body, so to speak, but the cell recedes, leaving them behind. By this
arrangement the terminals of the future fibers become definitely estab-
lished, all increase in length taking place in the opposite direction, toward
the pulp. While the active enamel-forming cells are present some little
time prior to the odontoblasts, calcification of the enamel does not take
place until after a definite cap of dentin has been formed, imprisoned in
which are the terminal branches of the fibers. Therefore the fact that
this cap of dentin is formed first, and this is not a question in dispute,
with the fibers or cell processes securely encapsulated within it, would

376 HISTOLOGY.

seem to be sufficient evidence to qualify the statement that the dentinal
fibers do not penetrate the enamel. The examination of very many
sections of young growing teeth exhibits the fact that the early formed
dentin and enamel will separate bodily, leaving a positive clear line of
separation and a surface absolutely devoid of anything resembling the
prolongation of the fibers extending from the surface of the dentin.

Interglobular Spaces. — In that part of the dentin which immediately
underlies the cementum numerous intercommunicating, irregularly
branched spaces are found. These are known as the interglobular spaces
(Fig. 288). On account of the granular appearance which this portion
of the dentin exhibits under low magnifying power, Tomes has desig-

Fig. 289. — So-called Interglobular Spaces in Dried Section of Dentin, x i°o.

nated it as the "granular layer.'''' The granular layer is also found upon
that portion of the dentin which underlies the enamel, but in this region
it is far less marked. Many of the dentin tubes have their endings in
these spaces. While the interglobular spaces are most numerous near
the peripheral portion of the dentin, they are by no means confined to these
parts. They are present in all parts of the dentin, but not so closely
associated, and may be observed, when a dried section of dentin is exam-
ined, as spaces with irregular outlines and sharp-pointed processes ex-
tending in various directions (Fig. 289) . The term ' ' interglobular spaces ' '
becomes partly a misnomer when the so-called "spaces" are more care-
fully examined. In normal dentin the "spaces" are filled with a soft,
living plasma, having a structural arrangement similar to the general

DENTIN.

377

matrix of the dentin, and it is only in a dried specimen that a true space
is found by the shrinking or shriveling of the organic contents. The
interglobular spaces forming the granular layer, which are much more
numerous, but of smaller size, than those found in the body of the dentin,
are also filled with a soft living plasma, and they communicate, on one
hand, with the dentinal fibers, and, on the other, with the lacunas and
canaliculi of the cementum. According to Sudduth, the interglobular
spaces (so called) are occupied by masses of calcoglobulin which have
not become fully calcified.

Dentinification. — The dentin bulb, or papilla from which the dentin
is forced, having already been described in Part I, the process of calcifi-

;^*

Pulp-cells K's!.; 'SrSsJr- 58. •' c*»wr

Odontoblasts

<!>

Odontoblasts 4?< n . o*~ ]3

Calcified Dentin

Uncalcified Dentin

| BP>

Fig. 290. — Pulp and Forming Dentin from an Incisor Tooth. (After Gysi.)

cation will at once be taken up. It will be recalled that calcification of
the dentin does not begin until the dentin papilla has developed to the
form and size of the dentin of the future tooth-crown. When this has
taken place, there is generated upon the surface of the papilla a modified
form of connective-tissue cells called odontoblasts (Fig. 290). These
cells, which are arranged in a single row upon the exterior of the papilla,
vary in form according to their activity. When most active, they are
broadest at the extremity directed toward the interior of the papilla.
Proceeding from a single odontoblast there may be one or more processes,
which are supposed to eventually occupy the tubes of the dentin, as the
dentinal fibers. These cells each contain an oblong nucleus, which occu-
pies the extremity of the cell most distant from the dentin, but during

378

HISTOLOGY.

the period of greatest activity becomes elongated or pointed in the direc-
tion of the process. The odontoblastic cells, while actively engaged in
the calcifying process, are closely associated or crowded together, but
previous to this time there is more or less space between them, which is
filled with an indifferent tissue. The first layer of dentin being formed
upon the surface of the papilla, it will be observed that all additions to
its bulk take place from within (the reverse being true of enamel).

Fig. 291. — Section through Pulp and Forming Dentin.

As stated elsewhere, calcification of the dentin begins upon the
coronal extremities of the crowns, the cutting-edges of the incisors and
cuspids, and the summits of the cusps in the cuspidate teeth first receiving
their lime-salts. While the odontoblasts undoubtedly superintend the
calcifying process, the part taken by these cells appears to be somewhat
indefinitely determined. It is generally supposed that the lime-salts are
secreted under the superintendency of the odontoblasts. The secretion,
however, does not take place around the cells, and in that way completely

dextix. 379

encapsule them, but around their fibrils. While this is taking place the
odontoblasts remain free upon the surface of the pulp, and the fibrils
assume their places as the organic dentinal fibrils. As the body of dentin
becomes thicker, the odontoblasts are forced to recede, and in so doing
the fibers lengthen. The dentinal tubules are, of course, formed in a
like manner, the walls of the tube being first calcified from the secretion
of lime-salts by the fibrils; and as the fibrils lengthen by the increasing
thickness of dentin and the receding of the odontoblasts, the tubes also
lengthen.

The general character of the pulp cells at a time immediately prior
to the appearance of the ameloblasts is vastly different from the same
cells at maturity or after calcification of the dentin has taken place. The
reason for this is obvious, considering that the connective-tissue mass
does not assume its principal function until the odontoblasts are generated
about its periphery. While at a later period the cells of the pulp are
oblong in shape, with slender tail-like processes given off from each end,
we find the same cells in the early embryo (sixteenth to twentieth week)
spheroidal in outline and distributed as they continue to be, at irregular
intervals about the semi-gelatinous matrix. (See Fig. 291.)

When the periphery of the pulp is reached, a definite layer of cells is
present, corresponding in every particular to those of the interior, and
it is from these spheric bodies that the dentin- forming cells are derived.
While the odontoblasts are usually characterized as spindle- or flask-
shaped cells, this can only apply to the cells of later life, as those active
at the beginning of calcification do not partake of either of these forms.
Figure 292 (twentieth to twenty-fourth week) shows the first formed
odontoblasts actively engaged in their function of dentinification. It will
be observed that the cells, instead of being individualized as they appear
at a later period, now present a racemose arrangement, such a cluster
appearing about the entrance to each dentinal tubuli, which at this period
are widely separated and apparently without anastomosing branches.
The nearer the summit of the crown is approached, the less apparent is
this grape-like association of the cells, showing conclusively that it is
a primary condition.

After a definite thickness of calcified dentin appears about the surface
of the tooth-pulp, the character of the odontoblastic cells becomes mate-
rially changed (twenty-fourth week), but even yet they do not answer
to the description accorded them. The elongated, spindle-shaped or
club-shaped odontoblasts are without question found in connection with
the tissue only after calcification has progressed to a considerable extent;

3 8o

HISTOLOGY.

and while it does not appear possible to detect the minute processes which
penetrate the calcifying structure before this stage of the phenomena has
been reached, they have nevertheless existed from the earliest inception
of this specialized layer of cells.

In this connection the query presents itself in regard to the manner
in which the intercommunication between the dentinal fibers is established,
and the probable cause for the so-called interglobular spaces about the

■J)tv»U\».

1MB

Fig. 292 — Young Odontoblasts Attached to Forming Dentin.

periphery of the dentin. The former can probably be explained by an
examination of the peripheral cells of the pulp at a time immediately
prior to the beginning of calcification, when it will be found that these primi-
tive odontoblasts communicate with one another in a manner quite
similar to the canaliculi between the lacunae of the true bone, the connect-
ing processes being encapsuled within the substance of the calcifying
tissue.

Figure 293 is taken from a very thin section of a growing tooth at

DENTIN.

3*i

a time in which we would most naturally look for the appearance of the
interglobular spaces, and many such imperfections, if they be so classified,
are observed within the substance of the newly formed tissue.

Exceptions might be taken to the statement concerning the racemose
appearance of the early odontoblasts previously referred to, by claiming
the section to be one not directly through the long axis of the cells, or

FiG. 293. — Section through Crown of Growing Tooth of Lamb.

perhaps transversely through them. The examination of a number of
sections, one of which is shown in figure 293, shows the cells cut trans-
versely; the forming dentin at a appears with the tubuli squarely cut off,
showing the dentinal fibers confined, or rather appearing as though pro-
jecting from the lumen of the tubes. It will also be noted that the odonto-
blasts are irregular in outline, some of them being almost hexagonal, and

3 82

HISTOLOGY.

as the calcified tissue is approached, they gradually become reduced in
size and much modified in contour.

After a dentin cap or matrix of considerable thickness has made its
appearance and the enamel cells are about to assume their functional
activity, the odontoblasts for the first time begin to resolve themselves
into the elongated flask-shaped cells, thus answering the description
usually accorded them, as illustrated in figure 290. In fact it would

Fig. 294. — Odontoblasts in Transverse Section.

appear that they assume this shape only when the actual lime deposit
begins. If we examine the line of union between the dentin and enamel
during the early growth of these tissues, it will be ascertained that, not-
withstanding the dissimilarity of the two structures at maturity, there
appears at this line of junction a matrix which may be differentiated only
by the free extremities of the ameloblasts, as shown in figure 295. When
the enamel matrix begins to form, a faint line of demarcation between
the two may be observed, the difference in the appearance of the ground-

DENTIN.

3*3

work of the two structures being one brought out by differential staining,
that of the enamel taking the darkest stain.

It will be observed, therefore, that after the dentin germ has assumed
the exact size of the dentin of the future tooth, certain cells appear upon
its periphery, and under their superintendence a definite layer of dentin

Fig. 295. — Section through Pulp, Dentin, and Forming Enamel.
A, Calcified dentin; B, Pulp; C, Ameloblasts.

soon results. This first formed layer of dentin is definite and unchange-
able in location, and it has within its substance the minute processes from
the dentin-forming cells which are destined to become and really are the
terminals of the dentinal tubules. All who have given the subject of
dentin calcification careful consideration are practically agreed as to the
part which the peripheral pulp-cells play in the process. This is to the

384 HISTOLOGY.

effect that not about the body of the cells themselves, but around their
processes the lime salts are deposited. After a distinct layer of specialized
cells has become fully established upon the very periphery of the papilla,
the first change which takes place is a slight withdrawal of these cells
from this point, leaving behind slender hair-like processes which occupy a
portion of the space previously taken up by them, and about the ex-
tremities of the cells and their processes which are directed toward the
enamel organ calcified material is generated. Zone upon zone of calcified
dentin appears in this way, the body of the cell receding, leaving in its
wake its processes encapsuled within the calcified structure as the dentinal
fibers.

In connection with the primitive layer of dentin-forming cells, there
are usually described lateral processes passing from cell to cell, apparently
serving the purpose of communication between the cells. But these
have recently been shown to be simply a network of connective-tissue
fibers supporting the body of the cells. The theory of Andrews, brought
out some years ago in regard to the specialized layer of pear-shaped
cells — dentin corpuscles, as he termed them — may be accepted, and
these should be considered as having something to do with the process of
dentinification. The presence of these pear-shaped cells at the beginning
of calcification and during the continuance of this process can be easily
demonstrated, and if we accept them as being concerned in the process
of dentin formation, they might in a measure modify the function now
accorded the elongated club-shaped cells, the odontoblasts. It is ques-
tionable whether the odontoblasts alone are reponsible for the growth
of dentin; they undoubtedly control the actual process of lime deposit,
but the additional cells, no doubt, contribute to the structural make-up
of the tissue. It may be that by. modifying certain parts of the matrix,
the result in the general structure is the dentinal sheaths; this part of the
tissue being so markedly different from the bulk of the intercellular sub-
stance would lead us to believe that it was developed from specialized,
cells. Further, it is said that while the dentinal tubules are filled with
a living substance, this substance is not solely the product of the processes
of the odontoblasts. That there is a special distribution of non-medul-
lated nerve terminals as well as a rich plexus of blood-vessels about the
periphery of the pulp is unquestionable, and this supply is just as plentiful,
or perhaps more so, at maturity as it is at the beginning of calcification
when the dentin cells are most active. From this we might be led to
believe that this special blood and nerve supply to the periphery of the
pulp is not solely for the upbuilding of the dentin and therefore distributed

CEMENTUM.

385

to the peripheral cells, but also for the permanent welfare of the resultant
tissue, this being brought about by some circulatory system throughout
the tubules of the dentin.

Cementum (Fig. 296). — Investing the roots of the teeth is a sub-
stance which, both chemically and physically, is closely allied to bone.
This external covering is known as the cementum, and while generally
regarded as being confined to the roots of the teeth, by some it is considered

Lucunae

Canal-
iculi •

Fig. 296. — Cementum from Root of Molar. X 200.

to extend to and completely invest the crowns during the early part of
their existence, in this latter location being known as the enamel cuticle,
or membrane of Nasmyth.

Generally speaking, the cementum begins by a thin margin at the
neck of the tooth or cervical line. It may commence at the free enamel
margin of the crown, or it may slightly overlap this structure. It is
thinnest at the neck of the tooth, and gradually increases in thickness as
the apex of the root is approached. In teeth with closely associated
25

3 86

HISTOLOGY.

roots the cementum frequently extends from one root to the other, result-
ing in a firm, osseous union. Histologically considered, the structure of
cementum, like ordinary bone, consists of a gelatinous, basal substance,
combined with the salts of lime, and of numerous little hollow spaces —
lacunce. Branching in every direction from the lacunae are many minute
processes — canaliculi.

The Matrix. — The matrix is so nearly identical with that of bone
that it is with difficulty that they can be distinguished. By decalcification
it retains its form and structure, and by the intimate blending of organic

ISMHMti

Fig. 297. — Longitudinal Section through Root of Human Molar.
Incremental Lines of Cementum. X 30.

and inorganic substances it is provided with hardness, solidity, and
elasticity. Calcium salts and collagenous fibrils, united by a small
amount of cement-substance, in finer or coarser bundles, compose the
ground-substance, or matrix, of cementum.

Let us first take up the study of this tissue at different periods of its
existence, and in this manner learn of its character, its mode of develop-
ment, and the changes which take place as its growth proceeds. The
striated markings of the tissue have led to the belief that there are, during
the process of cementification, periods of activity and periods of rest or
little activity. An examination of the structure under low power (Fig.
297) shows the incremental lines placed, with more or less regularity, one

CEMENTUM.

387

beyond the other, and when thus studied adds much to the strength of the
theory of interrupted development.

Figure 298 is prepared from a developing deciduous incisor three
months after birth. At this period the developing organ is made up of
enamel and dentin alone, the process of cementification not yet being
under way. The establishment of the dentinal periphery, which surface
is unchangeable, provides a basis for the first layer of cementum generated
by the cementoblasts, which at this period are forming about the inner
wall of the tooth-follicle. In close proximity to the surface the inter -

Enamel

Surface of
Dentin

^ .J-

*<.-.•:

Fig. 298. — Section through Developing Incisor, Three Months after Birth. X 30.

globular spaces are observed somewhat widely distributed, and propor-
tionately large in size, resulting in a surface poorly calcified and forming a
ready attachment for the cemental tissue. Figure 299 .shows the process
of cementification under way, the section being prepared from a six-
month-old tooth. In an examination of the ground-substance of this
developing tissue there is an unbroken granular appearance, possessing
neither striations, fibers, nor cement-corpuscles. This appearance is
one which persists in the oldest or first-formed stratum, and is again
noticeable in the outermost or youngest stratum. While the oldest
stratum or strata retain this primary character, this cannot be said of

3 88

HISTOLOGY.

those subsequently laid upon it, for they successively develop in their
matrix the partially calcified cells and fibers from the formative tissue.

Figure 300, taken from a one-year-old tooth, shows a further advance
in the process of cementification. Many of the transverse fibers of the
alveolodental membrane are observed penetrating the developing tissue,
and will, at a later period, by their partial calcification, become a part of
its substance. Already there has been established an intimate blending
of the cemental tissues with the dentinal tissues through the medium of the
granular layer, and by the further calcification of the latter this union

/v*.t<

Fig. 299. — Developing Cementum, from Six-month-old Tooth. X 200.
a, Developing Cementum; b, Granular Layer; c, Terminals of the Dentinal Tubuli.

gradually becomes more thorough. Figure 301 illustrates three distinct
zones of developing cementum; the older unbroken granular zone at A,
now beautifully cemented to the granular layer; a second or intermediate
zone, B, having encapsuled within its ground-substance many of its
formative cells; and an outer zone, C, but recently laid down, showing
numerous, longitudinal, wave-like striations, emblematic of the cemen-
toblastic activity. In this outer zone the minute laminations disappear
as the tissue becomes more thoroughly calcified and the matrix gradually
partakes of the nature of the older tissue.

CEMENTUM.

389

The position occupied by the cementum on the root has much to do
with its character. In the region of the cervix the cement-corpuscles

Fibers of
Cementum

Older Strat

ranular
Layer

-.v

Fig. 300. — Section through One-year-old Tooth. X 60.

Fig. 301. — Developing Cementum, from Transverse Section of Bicuspid. X 100.

are few in number, and when present possess extremely short and irregular
processes. In the region of the apex the structure is much more complex

39°

HISTOLOGY.

in character, longitudinal striae, transverse fibers, cement-corpuscles,
and zones of apparently unbroken granular matrix all serving to this end.

To continue the study of this tissue let us examine in detail the
lamellae, the cement-corpuscles, and the cement-fibers.

The Lamella. — We are told that the lamellae are about the same in
number over all parts of the tooth-root, but that they are much thinner
at the neck than at the apex. In addition to this they are usually con-
sidered as running parallel, or nearly parallel, to the surface of the dentin.
While these statements might, and probaby do, describe the disposition

i ; 11 ^ • i in i— i * T

\fafc - - 71 ~- c -» j . ■■+ ■<* L.T

Fig. 302. — Transverse Section from Root of Bicuspid, showing Variation in the
Disposition of the Lamellae. X 40.

of the lamellae in young cementum, they do not apply with so much
certainty to the conditions after the adult period. The lamellae in the
region of the apex are not only of greater width, but are usually greater
in number than those occupying the cervix of the same root.

Figure 302 is prepared from a transverse section of an adult bicuspid
in the region of the apex, and shows how the disposition of the lamellae
may vary in thin, normal cementum. At A, which represents the granular
union of the cementum with the dentin, the incremental lines are ob-
served to follow the surface of the dentin. As the center of the area is
approached this regularity is much interfered with, some of the lamellae
being discontinued, others greatly thickened, while the field, taken in its

CEMENTUM.

39 1

entirety, exhibits anything but regularity in the laying down of the
different strata. This same condition may be observed in longitudinal
section. While the lamellae are usually characteristic of the cemental
tissue in general, they are seldom found in interdentinal cementum, or
that growth which takes place between roots, resulting in their fusion
(Fig. 303). This, of course, refers to the tissue as formed between closely
associated roots of an individual tooth, and not to that union which
sometimes takes place between the roots of different teeth. The inter-
dentinal tissue previously referred to appears to have many characteristics

Interdentinal
Cementum

Fig. 303. — Transverse Section through Fused Roots of Molar Tocth, showing
Interdentinal Cementum. X 30.

common^to itself; thus, the cement-corpuscles are peculiar in form, fibers
are few in number, and, as before stated, the lamellae are not decided.
Cement-corpuscles. — Many of the cementoblasts of the peridental
membrane, like the osteoblasts of the periosteum, become encapsuled
within the developing tissue, and persist as irregularly shaped spaces,
filled with a protoplasmic mass, and are known as cement-corpuscles.
These correspond to the lacuna: oj bene, but, unlike these, are very
variable in size, in form, and in the number and direction of their proc-
esses. Figure 296 shows a number of cemental lacunae and canaliculi.
In the majority of instances the body of the corpuscle will be found to be
oval or slightly oblong, with its long axis parallel to the surface; but it is
by' no means uncommon to find them very irregular in outline, with the

392

HISTOLOGY.

greatest diameter in the opposite direction. The processes are quite
variable in length and irregular in their course, and, while there is a
general disposition for them to extend toward the surface, they in many
instances radiate in various directions. All of these features are in con-
tradistinction to the lacunas and canaliculi of bone, which are placed
with much more regularity in the osseous matrix, the corpuscles being
oblong or cylindric in outline, with their processes about equally distrib-
uted in every direction, and uniting directly and positively with the
canaliculi of neighboring lacunae. As previously stated, the cement-

• /C^vv ,

Fig. 304. — Cement-corpuscles of Outer or Younger Strata. X 40.

corpuscles are very variable in outline, this difference in form appearing
to be much influenced by the part of the tooth examined. The younger
corpuscles (Fig. 304), or those associated with the outer strata, are usually
distinctly outlined and provided with delicate processes, the majority of
which are directed toward the surface. In the older strata the outlines
of the corpuscles are much more irregular, the processes short and
extremely clumsy.

The proportionate distribution of the corpuscles to the various parts
of the tooth-root is as follows: The innermost or oldest zone and the
outermost or youngest zones contain but few; in the intervening strata
they are most abundant, especially in the region of the apex, becoming
less numerous in passing crownward. In interdentinal cementum the

CEMENTUM.

393

corpuscles are somewhat regularly distributed throughout the ground-
substance adjacent to the granular layer, but near the center of this
confused mass of imperfectly calcified tissue they are seldom present.
When the interdentinal space is slight, peculiarly formed corpuscles are
often observed (Fig. 305), provided with a long, rod-like, central portion
or trunk, from which are given off numerous tree-like branches, the
terminals of which are frequently lost in the granular layer upon either
side.

Cement-fibers. — In a manner similar to that in which the cemento-
blasts become encapsuled within the developing cemental tissue forming

■'-- Y •»'f #-'4 V^i

Fig. 305. — Cement-corpuscles Common to Interdentinal Cementum. X ioo.

the cement-corpuscles, many of the fibers of the peridental membrane
undergo a like transformation, and are found in the tissue as more or less
imperfectly calcified fibers transversely disposed. By many writers these
filamentary, thread-like structures have been compared to the delicate,
net-like processes which pass through the concentric lamellae of bone,
serving to hold them together and designated as Sharpens fibers; but,
according to Black, these are the principal fibers of the alveolodental
periosteum, and, as already stated, become a part of the cemental tissue
during its evolution. In figure 306 the fibers are shown under high
power; A represents the primary or older stratum of the tissue, and it is
from the outer margin of this zone that the fibers first make their appear-
ance, passing more or less directly in the direction of the surface until the

394 HISTOLOGY.

next incremental line is reached, at which point they gradually disappear,
but recur in the succeeding lamellae. There is a marked disposition for
the fibers of each concentric lamella to keep within its borders, or, in
other words, to become individualized; but in many instances they pass
through from one lamella to another, and occasionally extend unbroken
through the entire thickness of the tissue. It occasionally happens that
the fibers are plentifully distributed to a region comprising three or four
lamellae, followed by a zone of similar proportions in which they are
entirely absent. The cement-fibers, considered as the partially calcified
residue of the principal fibers of the peridental membrane, would naturally

Fig. 306. — Transverse Section through Root of Molar, showing Cemental Fibers. X 300.

assume a general direction relative to their manner of distribution before
this change had taken place, and in most instances they are thus disposed.
In figure 307, taken from the center of a long axis of a growing bicuspid,
the disposition of the fibers, which are alone observed in the second
lamella, is slightly crownward. The inclination for the fibers to be thus
disposed is most pronounced in young cementum, but after middle life,
or at a period when the tissue has greatly increased in thickness, the
course of the fibers, even in the same locality, is greatly at variance.

In figure 300, also from a young tooth, the fibers are shown springing
directly from the peridental membrane, with their free extremities
penetrating this tissue. This illustration is prepared from a transverse
section in the cervical region, and the inclination of the fibers is such as

CEMENTUM.

395

to warrant the belief that they were some of those whose function it has
been to return the tooth to its normal position when slightly rotated upon
itself. Another class of fibers common to the cement-tissue are those
which appear to be grouped in bundles, springing more or less regularly,
at intervals, from the granular layer and penetrating the basement layer
of the cementum as though serving to tie this tissue to the periphery of
the dentin. In figure 308 a number of these bundles are shown at A, B,
and C. While the field is but a small proportion of the circumference
of the root, they are observed, under low power, to be distributed in a

like manner to all parts. These circumferential fibers, as they may be
called, are also observed in longitudinal section, being distributed with
considerable regularity throughout the whole extent of the root. They
are also present in the tissue at the earliest period at which its character
may be studied, the individual bundles at this time being proportionately
larger. These might be, and probably are, considered as prolongations
from the dentinal fibers, but it is doubtful if the true fibers of the dentin
are ever found penetrating the cementum.

Cementification. — We have seen in the study of the development of
the teeth, that the tooth-generating organs were confined in a closed sac
or follicle, and while the walls of this sac were not directly interested in
the calcification of the dentin or enamel, this cannot be said of the
cementum. Attention has also been directed to the fact that at the time

396

HISTOLOGY.

of the eruption of the crown of the tooth a portion of the root only is
calcified. As the growth of the root continues, the follicular wall becomes
closely adherent to it. Upon the inner face of this vascular membrane
a layer of osteoblastic cells (cementoblasts) is generated, and as a result
of the calcification of these cells the cementum is formed. It will thus be
seen that the process of cementification is but a slightly modified form of
subperiosteal bone development. At the beginning of cementum cal-
cification the diameter of the dentin of the root is as great as it will ever
be, all additions to its bulk taking place from within. But while the

Fig. 30S.

diameter of the dentin is thus fixed, the diameter of the root is increased
by the additional layers of cementum as they are deposited upon its
surface. As previously stated, a single layer of cementoblasts is first
formed in the membrane surrounding the root, these soon becoming
inclosed in a spherule of lime. By the time this has taken place another
layer makes its appearance, assuming all the characteristics of the first
formed layer. Other layers are formed in turn until the cementum
assumes its mature thickness.

The Dental Pulp (Fig. 309). — The tooth-pulp, or formative organ
of the dentin, occupies the central or pulp-cavity, and in the fully developed
tooth assumes a general outline closely corresponding to the exterior

THE DENTAL PULP.

597

of the organ.* Along with its primary function of generating the dentin,
it becomes the medium through which this structure receives its vascular
and nervous supply.

Histologically considered, the pulp may be described as a mucous-
like, protoplasmic matrix, containing delicate connective-tissue fibers
not formed into bundles and numerous nucleated cells, the latter being
especially numerous on the periphery of the pulp, or that portion which
comes in contact with the dentin. The cells are not closely enough

Enamel

Cementum

Dentin

Pulp

Blood-vessels

Fig. 309. — Longitudinal Section through Human Cuspid, showing Tooth-pulp.

{After Gy si.) X 10.

associated to form a complete tissue in themselves, but are found em-
bedded in a mucoid matrix, with always a definite space between them.
In general the cells are elongated or spindle-shaped, with a delicate,
hair-like process attached to either extremity. In the pulp-chamber the
cells vary somewhat in outline, in some instances being spheroid, in
others appearing as slender filaments, so that the cell proper can
scarcely be distinguished from its processes. A third class of cells

*The pulp not only occupies the central cavity in the tooth-crown, but the canals of the
roots as well; therefore the form of the pulp corresponds to the outline of the pulp-cavity,
already described.

39«

HISTOLOGY.

may be met with, from which three or more filaments are given off. As
stated, the distribution of cells varies considerably in different parts of
the pulp, this being true not only as regards numbers, but also as to the
relations existing between the cells. In the coronal portion of the pulp
the position assumed by each individual cell appears to be without regard
to the position of neighboring cells, while in that portion of the pulp
occupying the root-canals the cells are arranged parallel with the length

Fig. 310. — Transverse Section through Pulp. Blood-vessels and Nerves in Cross Section.

of the root. The cells are least in number in the interior of the pulp,
but gradually become more plentiful as the periphery is approached. (See
Cells of Dentin Papilla, page 257).

The Odontoblasts (Fig. 311). — The most active cells of the pulp are
those directly on its periphery, in contact with the dentin, and known
as the odontoblasts. The odontoblastic layer, otherwise known as the
membrana eboris, is composed of a single row of cells, each of which
contains, near the extremity most distant from the dentin, a well-defined
nucleus. They are large, elongated cells, each furnished with three sets

THE DENTAL PULP.

399

of fibers or processes — the dentinal process, the pulpal process, and the
lateral process. The dentinal process or processes — there may be more
than one present — communicate with the deeper-lying cells of the pulp,
while by means of the lateral processes the cells are brought into com-
munication with neighboring cells. The processes given off in the
direction of the dentin, or the dentinal processes, may be one for each cell,
in which case they are of considerable size, and are inclined to taper as

Dentin

Odonto.
blasts

Blood-
vessels

Fig. 311. — Pulp and Dentin in Longitudinal Section.

they enter the substance of the dentin. Again, a single cell may give off
a number of smaller processes in this direction. The odontoblasts vary
much in form according to their functional activity. Before the period
of dentinification they are spheroid or pyriform, during the period of
calcification the dentin extremity becomes somewhat flattened and
square, while in advanced years they again return to their primitive,
rounded form. Covering the entire surface of the pulp like an epithelium,
the odontoblasts are especially closely associated at the end nearest the
dentin, forming an unbroken layer, while the pulpal extremities are
inclined to assert their individuality by disassociation.

400

HISTOLOGY.

Blood-vessels of the Pulp. — The pulp is richly supplied with blood-
vessels, forming networks extending principally in a direction parallel to
the long axis of the tooth, and finally terminate in a capillary plexus
closely associated with the odontoblastic layer. The veins of the pulp
are ordinarily somewhat larger than the arteries, and form numerous
anastomoses. This organ appears to be destitute of lymphatics — at

Fig. 312. — Section through Pulp. Fig 311 in Transverse Section.

least, none are known to occur in its substance. The blood-vessels of the
pulp are provided with a longitudinal layer of thinly distributed muscular
fiber, but otherwise the walls of the vessels are noted for their delicacy.

Nerves of the Pulp. — After entering the apical foramen either by one
large trunk or by two or more minute ones, the fibers pursue a parallel
course, breaking up but little or giving off but few fibers in that portion
of the pulp confined to the canal. When the expanded or coronal portion
of the pulp is reached, numerous subdivisions occur which are distributed
in every direction, and ending in a rich plexus beneath the odontoblastic
layer, or membrana eboris. In the body of the pulp the fibers are

THE DENTAL PULP.

40I

medullated, but those occupying the periphery are non-medullated and
supposed to pass into the dentinal tubes. While this latter hypothesis
is in all probability correct, such a distribution of the germinal fibers has
never been definitely demonstrated. Two investigators (Ball and
Magitot) claim to have partially satisfied themselves in regard to the final
distribution of the non-medullated fibers. The former states that he has
traced these fibers into continuity with the larger medullated fibers in the
deeper pulp-tissue, and claims to have found them passing through the

Main Nerve-
trunk

Main Blood-
vessels

Branching of Main
Nerve-trunk into
Single Fibers

Branching of Main
Blood-vessels into
Capillaries

Fig. 313. — Distribution of Blood-vessels and Nerves to the Pulp of Human Molar.

{After Gysi.) X 20.

membrana eboris, beyond which point they assumed a direction parallel
to the dentinal tubules. This theory is controverted by Magitot, who
claims that the dentinal fibers are, in a measure, themselves prolongations
of the nerves, being so constituted through the medium of the branched
stellate cells which lie immediately beneath the membrana eboris, and
by which the nerves are made continuous.

Nasmyth's Membrane. — Nasmyth's membrane, otherwise known
as the enamel cuticle or persistent dentinal capsule, is an exceedingly thin
and peculiarly indestructible structure, entirely covering the enamel.
As to the presence of this membrane, which can be demonstrated only by
chemic detachment, there appears no doubt, but in regard to its origin
and definite structure much difference of opinion has been expressed.
26

402

HISTOLOGY.

By some writers (Tomes and Magitot) it is maintained that it is continuous
with, and similar in structure to, the cementum covering the root, being
an extension of the outermost layer in the region of the neck of the tooth;
and, in view of the fact that lacunas are found in its substance, this theory
would appear to be correct. On the other hand, it is considered to be a
product of the epithelium (Huxley and Kolliker) and in no manner con-
nected with the cementum. In the opinion of the author, it would be
difficult to understand how the theory advanced by Tomes could be
accepted. During the entire period of saccular development the crown

Alveolar Wal

Cementum

Alveolodental 3BH

Membrane BB

Blood-vessels of
Alveolodental Mem-
brane in Trans-
verse Section

rS*^

Dentin

Fig. 314. — Transverse Section through Root of Human Incisor and Surrounding
Alveolar Wall, with Alveolodental Membrane Intervening. X 40.

of the tooth is in close relationship to the enamel organ, this structure
intervening between the forming enamel and the wall of the tooth-sac,
from which the cementum is developed. It would, therefore, appear
that this membrane is generated from the external epithelial layer of the
enamel organ by a change in the character and form of these cells. Sud-
duth attributes its formation to a metamorphosis of the ameloblastic
layer, the prismatic cells assuming a horizontal direction. The amelo-
blasts are observed to be prismatic in form up to the point at which the
enamel prisms are yet unfinished, but as the surface is approached they
are observed to shorten and widen, and near the gum-margin they
assume a longitudinal direction instead of being at right angles to the

ALVEOLODENTAL MEMBRANE.

403

body of the crown. Mrs. Emily Whitman has devoted much time to the
study of the development of mammalian teeth, and appears to be of the
opinion that the cuticula dentis is the result of a change in the form and
character of the enamel cells, this metamorphosis taking place either
before or after calcification of the underlying tooth-tissues. Nasmyth's
membrane shows many characteristics which differ from those of the
body of enamel subjacent to it, serving as an indestructible, highly
polished surface-capping to the enamel prisms. The indestructible
nature of this membrane by reagents would appear to indicate that in

Fig. 315. — Section through Root of Tooth, Alveolodental Membrane, and Alveolus
a, Alveolus; b, Blood-vessel; c, Alveolar Portion; d, Dental Portion; e, Cementun .

structure it is closely akin to the structure lining the dentinal tubules,
the lacunae, etc.

Alveolodental Membrane (Figs. 314 and 315) . — As a general descrip-
tion of this membrane has already been given in Part I, it alone remains
to treat of its histologic character, which may best be accomplished by first
referring to the duties which it has to perform. These may be divided
into three classes — functional, physical, and sensory. The functional
office is accomplished through its cellular elements — the osteoblasts and
cementoblasts; the physical office is performed by the fibrous elements,
through which the tooth is fixed in its position; and the sensory office

404

HISTOLOGY.

through the abundance of nerves, which are richly distributed to all parts
of the membrane. We, therefore, find in this structure, besides con-
nective tissue, cells, fibers, nerves, and blood-vessels. The principal
cells, as already stated, are the osteoblasts and cementoblasts, but there
are also present fibroblasts and osteoclasts.

The osteoblasts, which are instrumental in the upbuilding of a portion
of the alveolar walls, are found lying against the bone, between the
principal fibers. These cells do not appear to be evenly distributed,
being numerous and crowded together in some parts, while others will

Epithelium

Connective
Tissue of
Gum

Alveolar

Wall

Alveolo-
— dental
Membrane

Cementum
of Root

Fig. 316.— Transverse Section through Root of Tooth, Alveolodental Membrane, Thin
Wall of Alveolus, and Gingival Tissue.

appear to be almost destitute of them. They are most plentiful in the
young subject, and seldom present at all in old age. In youth the
alveolodental membrane is thickest, and, as the building of bone occurs
on the inner wall of the alveolus, it can only progress as the membrane
becomes reduced in thickness. The osteoblasts are polygonal cells, in-
clining to the oval form, and vary greatly in size, with their longest
diameter at right angles with the surface of the forming bone. During
the period of the development of the young alveolar wall they are inclined
to be crowded together, and are frequently much distorted from pressure

ALVEOLODENTAL MEMBRANE. 405

upon one another. As age advances this condition becomes less pro-
nounced, and the cells separate into groups.

The Cementoblasts. — Stationed upon the opposite side of the mem-
brane, or that in contact with the root of the tooth, are another class of
cells — the cementoblasts — or those cells which are concerned in the
formation of the cementum. Like the osteoblasts, these cells are found
lying between the principal fibers of the root-membrane. They differ
in form from the osteoblasts, notwithstanding that they have a similar
function. Instead of the polygonal form common to the osteoblasts, we

Pulp

Dentin

Gingival
Tissue

Fig. 317. — Longitudinal Section through Root of Growing Tooth near the Cervix.

find these cells to be more or less flattened, with outlines somewhat
irregular. Extending from the body of the cell, which contains a well-
defined nucleus, are a number of irregular processes, which penetrate
the neighboring fibers or the interfibrous substance. Unlike the osteo-
blasts, the cementoblasts appear at all times to be evenly distributed
over the surface of the cementum, occupying all the space except that
taken up by the fibers as they leave the cementum. As to the develop-
ment of the osteoblasts and cementoblasts, they appear to be carried to
the fibrous meshes of the membrane by the blood as leukocytes or ameboid
cells, after which, by differentiation, they become fitted for the develop-
ment of bone or cementum, assuming their respective places against
the surface of one or the other of these structures.

406 HISTOLOGY.

Fibroblasts and Osteoclasts. — Fibroblasts and osteoclasts are also
present in the alveolodental membrane, the former for the purpose of the
increase or renewal of the fibrous tissue, the latter being functionally
concerned in the removal of a part of the alveolar walls to accommodate
the ever-varying position of the teeth, or acting in a similar manner upon
the cementum of the root. The osteoclasts, or giant-cells, are generally
inclined to the round or oblong form, and usually contain a number of
nucleoli. They vary much in size, and are seldom branched or provided
with processes. In addition to the four classes of cells already mentioned

Cementum

Blood-vessel

Alveolodental
Membrane

Gingivae

Fig. 318. — Longitudinal Section through Alveolodental Membrane, Gingival Tissue,
and Root of Tooth. Cervical District.

as being present within the meshes of the fibrous tissue of the root-mem-
brane, there is another class, present, however, during youth only, which
appears to be in course of development, and, therefore, without apparent
function.

The Fibers of the Alveolodental Membrane. — The principal fibers of
the alveolodental membrane are those which extend from the cementum
on one side to the alveolar wall on the other, and become firmly fixed
at either extremity by penetrating the calcified structures. The fibers
are all of the white, or inelastic, connective-tissue variety. It is by means
of the connective-tissue fibers that the actual attachment of the membrane

ALVEOLODENTAL MEMBRANE.

407

both to the bone and to the cementum takes place, the fibers passing
directly into the hard tissues, which they traverse for some distance,
being here known as Sharpens fibers.

The arrangement of the fibers is somewhat different over the various
parts of the root. In the region of the gingival margin they pass out
from the substance of the cementum, retaining their solid form or dividing
into fasciculi of finer fibers. In general the fibers lie parallel with one

Fibers

Fig. 319. — Section showing Fibers of Alveolodental Membrane, Attached to and Passing

Out from the Cementum.

another, deviating only to give place to blood-vessels and nerves. There
is some variation in the distribution of the fibers about the different
gingival surfaces. Upon the labial and lingual surfaces they pass out
directly into the fibrous tissue of the gum, and soon become lost in this
tissue. On the mesial and distal surfaces the fibers passing the lower
margin of the alveolar wall join the fibers of the neighboring tooth.
This disposition for the fibers to bend toward the adjacent tooth is first
observed at the various angles of the gingival margin. All about the

408

HISTOLOGY.

free border of the gum the fibers from the alveolodental membrane
assist in forming this tissue, which is covered by a dense epithelial coating
of moderate thickness, surrounded or surmounted by the peridental
fibers. As the border of the alveolar wall is approached, the fibers are
observed to pass under the proper tissues of the gum, and unite with the
outer periosteal layer overlying the outer alveolar wall. The fibers
immediately within the alveolus are slightly inclined in an apical direction,
while those occupying the central portion of the membrane, or that
midway between the apex and the gingiva, pass nearly straight across

Alveolus

Cementum

Pulp

Dentin

Fig. 320. — -Transverse Section through Growing Roots and Alveolus.

from the cementum to the bone. It is in this locality that the largest
and strongest fibers are found. As the apex of the root is approached
the inclination of the fibers is crownward from the cementum to the
alveolar wall. In this situation the single fibers are inclined to break up
the fasciculi. Immediately surrounding the apex of the root the fibers
are irregular during youth, but are disposed more regularly or fan-like
in older subjects.

While this account briefly furnishes a description of the distribution
of the fibers in various locations, and is in most instances correct, they
occasionally vary from this arrangement. While in most respects the
fibers of this membrane closely resemble the corresponding fibers of

ALVEOLODENTAL MEM3RANE. 40Q

attached periosteum, they possess some peculiarities. It might be
supposed that the fibers passing out from the cementum would in some
way differ from those springing from the alveolar wall, but, with the
exception of being somewhat less in size, they are otherwise of the same
character.

Interfibrous Elements. — Besides the various forms of cells, blood-
vessels, and nerves, there is present in the alveolodental membrane an
interfibrous tissue. This tissue is principally composed of the fibroblasts
belonging to the principal fibers, and other fibroblasts accompanied by
delicate fibers which appear to be independently distributed. This
interfibrous tissue, which is thus seen to be ordinary fibrous connective
tissue, appears to pervade the entire membrane wherever sufficient space
is found to permit of its presence. In some parts of the membrane this
tissue appears to be more plentiful than the principal fibers themselves.
The interfibrous tissue also forms an investment for the blood-vessels
and nerves in addition to the tissues properly belonging to their walls.

4io

HISTOLOGY.

Fig. 321. — Evolution of the Face. {After Haeckel.)

CHAPTER VII.

Embryology of the Mouth and Teeth.

The nourishment required for the growth of the embryo and its
maintainance during the earliest part of its development, and in higher
animals during the whole of this period, is supplied either from the
mother by means of a placenta, or it is drawn from the supply of concen-
trated food-material, stored up for that purpose in the form of yolk in
the egg. The formation of the future digestive organs of the mature
organism begins, however, already at a very early stage of the development
of the embryo. The essential epithelial parts of the digestive canal
are derived from the entoderm, while the more auxiliary parts, such as

muscles, connective tissue, blood-vessels, etc.,
which are not present in the lower animals,
but become more and more conspicuous as we
advance in the scale of animal organization,
are derived from the visceral layer of the
mesoderm. At first, the primitive gut presents
a shallow, wide groove, but it becomes grad-
ually deeper by a folding up of the sides; the
edges approximate more and more, and when
finally a union has taken place, there is a tube
formed — the so-called primitive digestive tract.
While the latter still has for a time an outlet
in the form of the vitelline or umbilical duct,
the tube is closed anteriorly, as well as pos-
teriorly. As development proceeds, there becomes noticeable a depression
of the ectoderm at the anterior end of the tube, opposite the ventral side of
the latter, and here ectoderm and entoderm lay closely attached to each
other, forming a thin membrane. This point of contact of the two layers
is known as the oral plate. Soon the plate ruptures, and thus an anterior
aperture of the gut, the oral sinus or primitive mouth, is formed.

The gradual transformation of the primitive aperture into the
permanent mouth and face is brought about mainly by the development
and transformations of its boundaries, and are, in brief, the following:
About the end of the third week, a series of conspicuous elevations or

411

Fig. 322. — Diagram showing
the relation between Ectoderm
and Entoderm in the Mouth of
a Mammalian Embryo.

a. I. and p. I., Anterior and
posterior lobes of the hyphysis;
m. t., medullary tube; ph.,
pharynx; o.p., oral plate; x
and y, ectoderm which pro-
duces the lip and teeth of the
lower and the upper jaw, re-
spectively. (Stohr.)

412

HISTOLOGY.

processes are developed around the opening, from the front and the sides.
The one coming down from the head is called jronto-nasal process; those
coming from the sides are arranged in form of parallel bars and are
known as arches. There are five pairs of arches; in fishes they give rise
to the formation of the gills, and the individual pairs are separated from

Fig. 323. Fig. 324.

Fig. 323. — Head of a Young Dogfish. (Slohr.)

Fig. 324. — Head of a Human Embryo of 10 mm. It Illustrates the Phylogenetic Rela-
tion of the Visceral Arches. (Slohr.)
g.c, Gill cleft; m, mouth; n, nasal pit; c.s., cervical sinus; g. c 2 , second gill cleft; h, hyoid
arch; md., mandibular arch; sp., (spiracle) auditory groove.

one another by slits. In human embryos there is no communication
between the inside and outside of the arches, but there are distinct
furrows which serve as lines of demarcation between the individual
bars. On account of these formations having connection genetically
with air-absorbing viscera, the following names are given: Visceral or
branchial arches, inner visceral furrows or pouches, external visceral
furrows or clefts. From the accompanying illustrations it can be clearly

Median Portion
of the Palate

Dental Ridge

Lateral Portion
of the Palate

Fig. 325. Fig. 326.

Fig. 325 and 326 Present the Development of Various Parts of the Palate. (After His.)

seen that only the frontal process and the first visceral arch participate
in the formation of the permanent mouth and face; the other four arches
give rise to parts, the consideration of which is beyond the scope of this
book. In regard to the details of the formation of the individual parts,
the following statements may be made:

THE ORAL CAVITY.

413

The Buccal Cavity.— The early appearance of the entrance to the
alimentary canal is found in the formation of an open cavity bounded by
the primitive maxillary processes above and mandibular arch below.

The cavity thus formed is the common buccal space, the upper
portion being the respiratory or nasal section, while below is the true
mouth. The cavity of the mouth, as such, does not exist until these two
are completely separated by the palatal plates forming the future roof
of the mouth.
* Figure 327 shows a vertical transverse section through this common

Nasal Cavity

Dental Ridge.
Upper Jaw

Dental Ridge,
Lower Jaw

Nasal Cartilage

Buccal Cavity

Tongue

Dental Ridge,
Lower Jaw

Fig. 327. — Vertical Transverse Section through Head of Human Embryo, about the

Tenth Week. x 30.

buccal cavity. At this early period the lateral walls and floor of the
mouth are manifest by certain cellular elements, but the roof of the
cavity, as already stated, is not complete until the palatal plates, now
separated by the tongue, grow inward and unite at the median line.

The Oral Cavity. — The Roof of the Mouth. — When these two
processes which arise from the mesoblast unite at the median line, they
establish a permanent horizontal septum, dividing this part of the stomo-
deum into a respiratory or nasal section and an oral section, the mouth.
The cells entering into this part of the fetal head at this time (eight to
tenth week) are of three varieties, being connective-tissue cells, cartilage

414

HISTOLOGY.

cells, and epithelial cells, the latter being distributed in a layer of varying
thickness over those parts destined to become a part of the lining mem-
brane of the mouth.

In figure 328 (twelfth week) the superior maxillary processes are
shown united and the permanent separation between the mouth and nasal
cavity established. This embryonal bridge is for the most part made up
of connective-tissue cells, about isolated bundles of which osteoblasts
arrange themselves, resulting in the production of two intermembranous
bony plates. *

Nasal Cavity

Dental Ridge

Tongu

Cartilagenous Septum
of Nose

Den al Ridge
Oral Cavity

Meckel's Cartilage

Dental Ridge

Anlageof Lowei Jaw

Fig. 328. — Vertical Transverse Section through Head of Human Embryo, about the
Twelfth Week, showing the Single Buccal Cavity Transformed into the Oral and Nasal
Cavities. X 30.

By the fourteenth week a further advance in the generation of the
hard palate is noted, the septum now being largely composed of calcified
tissue. The disposition for these primitive bony plates to exist as separate
and distinct processes is exemplified at the median line by a definite
separation formed by the connective-tissue sheath from which they are
derived. Covering the surface of the hard palate there now appears a
thin layer of mucous membrane.

The Floor of the Mouth. — Having thus briefly noted the evolution
of the roof of the mouth, let us next consider the floor of the cavity, the
tongue and its attached muscles, together with considerable glandular

THE FLOOR OF THE MOUTH.

415

tissue making up the bulk of this district. Figure 329 is a vertical
transverse section through the floor of the mouth about the tenth week
in the human fetus, or at a period somewhat later than that shown in the
previous illustration. An examination of the parts in general at a time
prior to this is of little value, save the early preparation for the develop-
ment of the teeth, which will be referred to later. The tissues and

Fig. 329. — Section through Base of Tongue and Lower Jaw. X 40.

organs here shown will be recognized as the tongue (A), the glandular
tissues (B), the forming jaw (C), with developing tooth-germs at D D.

The tongue appears on the floor of the mouth between the thirtieth
and thirty-sixth days as a bud from the mesoblast covered by a layer of
cells of epiblastic origin. The muscle-fibers, be they intrinsic or ex-
trinsic, are all of the striated variety. In a very short time, and at a
comparatively early period, the tongue becomes an independent organ,
presenting most of the characteristics common to it after birth. Not a
small portion of the floor of the mouth is made up of another class of

4i6

HISTOLOGY.

tissue which, although eventually a distinct organism, is composed almost
entirely of epithelium. These cells, together with the connective-tissue
cells, and eventually blood-vessels, unite in the production of a true
salivary gland, the sublingual. Figure 330 shows the early character of
the tissue, together with its relation to surrounding parts. The section
is one from the region of the premolars, and is bounded above by the
tongue, laterally by the borders of the jaw, and below by libers of the

Fig. 330. — Section of Sublingual District. X ioo.

mylohyoid and digastric muscles. Three distinct lobes or sections of
the gland are observed, the two largest being separated by a reticular
network of connective tissue.

The general character of these developing glands even at this early
period (about the twelfth week) appears to be very similar to the matured
organ, being composed of a number of small tubes emptying into a single
duct, constituting a gland of the compound tubular variety.

Let us next give some consideration to the embryology of the mouth

THE FLOOR OF THE MOUTH. 417

in its entirety; and to do this, it is necessary to make sections of the parts
in various directions.

The growth of the cavity is usually studied, and probably to the
best advantage, by vertical transverse sections, and attention will first be
called to a number of sections made in this way, beginning at the lips and
passing backward through the incisor region, and finally through the
districts occupied respectively by the cuspids and molars. The period

k^y

Fig. 331. — Embryonal Labial Mucous Membrane.

at which such an investigation is made has much to do with the character
of the tissue involved, but the time best suited to the purpose is included
between the fortieth and sixtieth days. At this time nearly all the tissues
making up the organs and parts which enter into the construction of the
cavity have advanced to such a degree of perfection that the investigation
may proceed with considerable satisfaction.

Figure 331 shows a cross-section through one of the primitive labial
folds about the period named. Little is to be observed in this district

4i8

HISTOLOGY.

at this early period except the simple cells of three varieties which serve
to make up the parts, but attention is at once attracted to the abundant
thickness of the epithelium given to the lip.

If a section be made somewhat to the distal of that previously shown,
a marked change in the relationship existing between the various cell
layers is observed in a body of cells of another character, those which are

Fig. 332. — Section through Base of Jaw.

destined to become the cartilage of Meckel, and about which the younger
layer of cells of mucous membrane are observed outlining a new district.
If a section be made through this same location, say about the
forty-eighth day, a vast change in the appearance of the parts is noticed
(Fig. 327). The buccal walls of the mouth have in a measure become
complete by a union of the upper and lower sections, the union at this
time being accomplished through the agency of the embryonal epithelium.
A cartilaginous nasal septum has made its appearance, and active prepa-
ration for the ossification of the maxilke is apparent. In the center of the

THE FLOOR OF THE MOUTH. 419

section is a distinct body of cells forming Meckel's cartilage, and early
preparations for the growth of the teeth may be seen at a by a dipping
down of the surface epithelium.

A transverse section through the same district about the sixtieth
day (Fig. 333) shows all the parts strongly differentiated. Ossification
has taken place to a considerable extent in the lower jaw, the two halves
being at this period, and for some months afterward, separate and distinct.

Fig. 333. — Section through the Wall of the Mouth of an Embryo, Sixtieth Day.

Many muscle bundles are observed beneath the jaw, and beyond these
the integument with its numerous blood-vessels and nerves, most of which
are seen in transverse section. A cross-section upon the same subject
about the sixtieth day in the region of the cuspids finds the tissues and
organs advanced to a certain degree of perfection. (See Fig. 329.) The
tooth-germs of the cuspid teeth have their crowns outlined by the cells
composing them; the tongue with its complex muscular arrangement
has become a specialized and independent organ, while beneath it we see

420

HISTOLOGY.

that product of the epiblast, the glandular structure, so plentifully supplied
to the floor of the mouth in this locality.

Passing further back into the region of the molars, the appearance
of the parts does not differ to any marked degree from that in the cuspid
district, except in the general distribution of the muscular fibers of the

Fig. 334. — Longitudinal Section through Chin of Embryo Lamb.

tongue, and the appearance of the submaxillary gland, here appearing in
three distinct lobes or parts.

It has been previously stated that sections made in the direction of
those already considered are usually employed to study these parts, but
much is to be gained by supplementing these with sections made in other
directions.

Meckel's cartilage.

421

Figure 334 shows a longitudinal section, or one made from mesial to
distal through the lower jaw at or near the median line, the parts included
within the field being the labial folds at A, the mandible at C, the tongue
at D, and a tooth-germ at E. An examination of the lips shows them to be
covered with a varying thickness of embryonal epithelial cells which are
continued backward over the future alveolar ridge and thence to the
hard palate above, or over the floor of the mouth and the surface of the
tongue below.

Meckel's Cartilage. — One of the earliest products of the mesoblast
is that which results in the production of Meckel's cartilage, which is

Fig. 335. — Diagram to Illustrate the Metamorphosis of the Visceral

Arches during Development.

I, Presents the first visceral arch, it gives rise to Meckel's cartilage, which

becomes transformed into the mandible. (After Wiedersheim.)

closely associated with the growth and early support of the lower jaw.
In the beginning, as already pointed out, the mandibular and hyoid arches
resemble one another, but soon after they become fully established they
take on different functions, and with this become dissimilar. The first
appearance of this cartilage as a distinct body of cells is found about
the middle of the second month, and when a transverse section of the
jaw is made for the purpose of studying its location and environments
(see Fig. 336), it is found near the base of the fetal head, considerably
below and to the outside of the base of the tongue. At mid-jaw it appears
as a circular body of cells separated from the surrounding parts by a
distinct layer of elongated cells. Even at this early period a portion of the
bony structure of the jaw is outlined by an aggregation of connective-
tissue cells, and the forming cartilage appears to subserve the purpose of

422

HISTOLOGY.

controlling the outline of the future jaw. The bow-shape of the cartilage
is manifest as we pass toward the symphysis by the lateral halves approach-
ing each other (Fig. 337), but the circular character of the cartilage in
cross-section is still retained.

Figure 338 represents a section through the symphysis about the
eighteenth week, and shows the two halves of the cartilage closely asso-

Fig. 336. — Section through Base of Lower Jaw, showing Meckel's Cartilage.

ciated, but not united, the separation being by a layer of connective-
tissue cells passing between the two. It will be noted also that the
cartilage, instead of being near the base of the jaw as in figure 337, now
appears near the floor of the mouth.

Figure 339 shows the relations existing between the two halves of
Meckel's cartilage and the growing mandible at the median line (a).

Meckel's cartilage.

423

It also illustrates how little the development of the bone is dependent upon
the cartilage, the growth of the former being in this district far below
and apparently distinct from the latter. Here, as in the upper jaw, the
periosteal cells from either side are observed to unite at the symphysis and
pass as a somewhat thickened layer between the two bones, the only
difference in the final change which takes place between the two being
that in the upper. jaw a suture results, while in the lower jaw a layer of

Fig. 337. — Section through Lower Jaw. M. C. Meckel's cartilage.

solid bone is formed. The character of this cartilaginous framework as
well as the cells which divide the two halves is shown in figure 340, the
cartilage cells being oblong or cylindrical, with a bountiful supply of
intercellular substance, while the connective-tissue cells are oblong or
spindle-shaped.

As soon as ossification in the jaw takes place to any extent, the
cartilage begins to atrophy, that portion lying next to the jaw degenerating

424

HISTOLOGY.

Fig. 338. — Meckel's Cartilage (M.C.) at the Symphysis.

Meckel's cartilage.

425

first, so that by the tenth or twelfth week it has entirely disappeared, but
before this takes place we find it surrounded by the periosteum, and
finally completely inclosed within the bone.

Figures 340 and 341 show the character of the cartilage cells about the

Fig. 339. — Ossification of the Mandible at the Median Line.

time that they are beginning to atrophy. It will be observed that the cells
are inclined to a change in form, and that they are proportionately larger
with large nuclei and nucleoli. A represents the district nearest the jaw,
and the cells in this region have already lost their characteristic outline.

426

HISTOLOGY.

Fig. 340. — Section of Meckel's Cartilage at Median Line.

Meckel's cartilage.

427

Fig. 341. — Cartilage cells in the beginning of atrophy.

428

HISTOLOGY.

FURTHER CONSIDERATION OF TOOTH
DEVELOPMENT.

One phase of the subject to which special attention will be given in
this chapter is that which denotes the period at which the various events
take place. This is a part of the study which is very difficult to deter-
mine, and it would appear, for this reason, if for no other, that investiga-

FlG. 342. — Transverse Section through Primitive Jaws of Human Fetus.

tors of recent years have been perfectly satisfied to accept the results
arrived at by their predecessors without any apparent effort to qualify the
deductions. It has for a long time been conceded that the primitive
changes which ultimately result in the formation of a tooth-germ are
first noted in a heaping up of the epithelial cells over the district repre-

TOOTH DEVELOPMENT.

429

senting the surface of the future jaw. While in many instances this is
true, there are reasons why it cannot be considered an essential feature.
In the first place, such a condition is not always present, as shown in fig-
ure 342, a transverse section through the primitive jaw of a human fetus
about the fortieth day. The tooth bands at A and B have penetrated
the submucous tissue for a considerable depth, but the surface epithelium
does not show a greater thickness at these points than it does over the
general surface of the cavity.

73.

Fig. 343. — Transverse Section through Lower Jaw, Human Embryo, Sixtieth Day.

Figure 343 shows a section made in the same direction upon a human
embryo about the sixtieth day, and while the tooth band (A) has pene-
trated the embryonal connective tissue to a greater depth, there is yet no
increase in the thickness of the epithelium, but rather a disposition for
the parts to become depressed. Another reason why the heaping up of
the superficial layer of cells forming the embryonal mucous membrane
should not be considered the first sign of the preparation for tooth develop-

430 HISTOLOGY.

ment lies in the fact that these cells are not directly interested in the
process, but that the inflection of cells which results in the formation of
the tooth band results from the deep or infant layer of cells known as
Malpighi's layer, as shown at B (Fig. 343).

There is no question but that the location from which the section is
taken has much to do with the character and thickness of the older layer
of epithelial cells, and that they do at certain points constitute an epithe-

FlG. 344. — Section through Tooth Band of Human Embryo.

Hum exceeding in thickness that of other parts of the cavity, but this
condition most frequently occurs after the enamel organ has assumed
definite proportions.

After the formation of the tooth band, which, it must be remembered,
encircles the entire jaw in the form of a well-defined body of oval epithelial
cells from the infant layer (shown at A in cross-section, Fig. 344), the
next step in the process is one which concerns the location for the individ-
ual buds for the enamel organs of the various teeth, and the approximate
time at which these appear.

TOOTH DEVELOPMENT.

43 1

In the human subject we find ten such spots appearing upon the
lamina given off from the lingual face of the tooth band. These do not
appear, however, at the free extremity of the band, but at some little
distance toward the surface from this point, as shown in figure 345. In
this section the tooth-germ is severed from the surface epithelium, but
this is not a true condition at this period, as it still retains its connection
with the surface by a narrow band of cells, the neck of the enamel organ.

Fig. 345.-

-Tooth-germ, Embryo Lamb, Corresponding to Sixtieth Day (Human).

X 40.

Two distinct classes of cells are now (sixtieth day) concerned in the
process of tooth development, those at A being of epithelial origin and
forming the future enamel organ, while at B an aggregation of cells from
the mesoblast provides for the generation of the pulp and dentin. At C
the narrow band of cells which should continue to the surface is shown,
while the free extremity of the same body of cells at D will persist and
eventually become the germ for the succeeding tooth. In regard to the
time at which the buds for the various teeth appear, it might be expected

43 2

HISTOLOGY.

that the same variation which follows the development and eruption of
the teeth throughout would obtain, but such is not the case, the buds for
the deciduous incisors appearing about the sixtieth day, while the germs
for their permanent successors are but little later in forming.

By referring to figure 334, which shows a longitudinal section through
the lower jaw about the twelfth week, the deciduous incisor is seen well
outlined by its formative cells, while immediately to the lingual appears a

Fig. 345. — Tooth-germ, Premolar, Embryo Lamb. X 40.

section of the germ for the permanent cuspid. Notwithstanding this,
there elapses a period of several years between the eruption of these teeth.
The same relative progress will be noted between the first and second
teeth, be the subject human or otherwise; requiring many years to com-
plete dentition, in the former, while in most of the lower animals the same
process occupies but a comparatively short time.

The next stage in the development of a tooth to which attention will
be called is that in which the entire tooth-crown is outlined by the dentin

TOOTH DEVELOPMENT.

433

papilla and surrounded by its epithelial cap, the enamel organ. Such
an advance in the process is shown in figure 346, together with the sur-
rounding structure. When this stage is reached, the individual cells
of the tooth-germ are strongly differentiated, and the odontoblasts are
making their appearance about the summits of the cusps.

Fig. 347. — Section through Jaw of Embryo Lamb, in a District not Occupied by

a Tooth-germ. X 40.

Up to this time the cells present are those which result in the forma-
tion of but two of the calcified tooth tissues, but now there is a marked
disposition upon the part of the periosteum of the jaw to pass down by
the side of the enamel organ (A), this being the first indication of
the formation of the tooth follicle, the alveolodental membrane, and
cementum. At this period it will be observed that there appears in this,
28

434

HISTOLOGY.

the molar region, a "heaping up" not only of the surface epithelium, but
also of the connective tissue as well.

Figure 347 shows a section through the growing mandible taken
from a portion of the tissue not occupied by a tooth-germ. This is of
interest, first, as giving a view of the detached tooth band in cross-section,

FlG. 348. — Primitive Bud for Enamel Organ of Permanent Tooth, Human

Embryo, x 300.

at A; second, by showing the distribution of the periosteum to the interior
of the jaw to serve the double function of the future tooth-sac and alveolo-
dental membrane; and, third, the thickened epithelium with the underly-
ing tissue pushing into it.

Although the germ for the second tooth may be observed at a period
somewhat prior to this, a study of some of its characteristics is best made

TOOTH DEVELOPMENT.

435

at this time. The fact has already been referred to that this interesting
phenomenon occurs soon after or even simultaneously with that for the
first tooth, a portion of the primitive cord for the latter persisting as the
germ for the former.

In figure 348 the cells forming the primitive germ for the enamel
organ of one of the permanent teeth are shown highly magnified. It
will be observed that they are of the simplest epithelial character, and
that they are derived directly from the enamel organ of the pre-existing
tooth, on the one hand, while, on the other, they communicate with the
surface by a narrow band of cells. In this way it is for a time dependent
upon both of these parts for continuance and growth, but after a time
it, too, like its predecessor, severs its connection with the surface, but
remains intact with the epithelial cells of the former enamel organ until
these cells begin to atrophy.

Fig. 349. — Diagram of Various Forms of Hare-lip following the Lack of Union of Various
Processes Participating in the Formation of the Mouth.

The cells which made up this primitive germ are of three varieties:
the inner layer, or those derived from the epithelium of the enamel organ
of the first tooth, being small and spheroidal; those of the outer layer,
which spring from the surface epithelium, being proportionately larger
and cylindrical or oblong; while those which intervene are markedly
irregular in outline. In this respect— that is, in the character of the early
cell layers — the tooth-germs for the permanent teeth differ from those
of the deciduous.

The question of the origin of those teeth which have no predecessors
is one upon which there has always been more or less discussion, some
writers contending that they are derived directly from the oral epithelium
by a special generation of cells for each tooth, while others are of the
opinion that as the jaw grows backward certain changes take place which
result in the establishment of an epithelial fold or lamina, in every partic-

436 HISTOLOGY.

ular corresponding to the tooth band of the deciduous teeth. With these
two conflicting opinions in mind, a number of sections were made through
the extreme distal end of the jaw. The result favored the latter theory,
for here the tooth band is seen similar in form and location to that ob-
served in the jaw in those locations from which succedaneous teeth result.

CHAPTER VIII.

Anomalies of the Teeth.

It has always been conceded that the dental organs of man are sus-
ceptible of much variation in form and structural arrangement, and
that frequently this variation is so positive that the organ is pronounced
anomalous in character. Just where the line of distinction between the
normal and abnormal should be drawn is a subject worthy of some con-
sideration. Some authorities define the word anomaly as a marked
deviation from the normal, while, in the opinion of others, a much broader
meaning is accorded it; and we find all those conditions which are in
themselves an irregularity from the typical structure or occurrence included

Fig. 350. — Anomalous Teeth.

in this category. Under the first definition a given structure or organ
is accorded a wide field for its normal existence, while under the latter
but slight deviation is necessary to classify it among the abnormal.

Upon first thought it would appear that the ability or inability of a
tissue or an organ to perform its special function should, in a measure,
decide the question of the nature of its being, and no doubt to a certain
extent this is true; but while the action of an organ or a part of the body
may, by observation, appear entirely satisfactory, it is only so at the
expense of other organs or tissues, and these in the course of time, by this
extra exertion, become hypertrophied or in other ways pathologic.

While this is especially applicable to those organs or tissues which
have a wide range of function, it may with a good deal of force be applied

437

438 ANOMALIES.

to the dental organs and their immediate environments. Anomalous
conditions in the teeth may originate in, or be confined to, one or more
of the tooth tissues, in any of which the structural disarrangement may
eventually result in the death or degeneracy of the part. Enamel mal-
formation is of such a character that it may be observed upon the surface
either in the form of a multiplication of cusps, or by an extra development
of the various ridges formed by pronounced folds of this tissue. But
probably the most disastrous anomaly of the enamel, and one frequently
responsible for the downfall of this tissue, is found in some defect of its
structural arrangement other than those just referred to. In some
instances the enamel rods of a given district, instead of being normally
distributed by assuming a direction principally at right angles to the long
axis of the tooth crown, are arranged without regard to the base or per-
iphery of the tissue, and we have as a result an anomaly of structure.
The question of normal and abnormal rod distribution now presents
itself, because in certain locations — i.e., the summits of the cusps —
an arrangement of the rods similar to that referred to is so common that
it may be considered a normal condition, while, if a like distribution was
found in other locations, the tissues should properly be considered
abnormal.

Malformed teeth, in respect to the number and forms of the
cusps present, are not alone confined to the enamel, but also to the dentin
which first records the tooth form on its periphery.

Anomalies in the general contour of the tooth crown are usually
confined to the incisors and third molars, both the dentin and the enamel
contributing to the deformity. Here the defect is usually so pronounced
that but little difficulty is experienced in properly classifying the organ.
One of the most frequent variations in form met with in these locations
is found in the peg-shaped or cone-shaped crown. If it were possible
it would be interesting to trace the development of such a malformation;
but with our present knowledge of this process in general, there is little
doubt as to its origin, the enamel organ failing to fulfill its early and pri-
mary function of moulding the tooth crown in the dentin papilla, the
responsibility for this resting in the special cells composing it, as well as
the so-called stellate reticulum, which, it is believed, exerts a controlling
influence over the form of the enamel cap.

While the organic defects of tooth crowns are numerous and varied,
those which are confined to the roots are most frequent, in many instances
interfering to some extent with the function of the organ. When a given
peculiarity is confined to this portion of the tooth, it is frequently difficult

SUPERNUMERARY TEETH. 439

to discriminate between the normal and the abnormal. Certain teeth
are recognized as normal when either a single root or two roots are pres-
ent, and the acceptance of this fact increases the difficulty of a proper
classification of its peculiarities.

In very rare instances do we find the roots of the cuspidate teeth
more or less crooked; yet, at the same time, many decidedly crooked
roots are considered within the natural law; while, on the other hand,
roots with but little more deflection are classed as anomalous.

Marked flexions of roots or crowns, cases of fusion or concrescence,
are usually so positive in character that an anomalous condition is at
once acknowledged. While tooth anomalies are usually referred to as
external, or as belonging to the hard tissues of the organ, they are not
infrequently found in the pulp or pulp cavity. This cavity, normally
following the external contour of the tooth, is subject to much variation
in outline and capacity, regardless of those changes which are incident to
the continuous process of dentinification.

Among these are a complete division of the pulp chamber; horn-
like processes penetrating the dentin in the direction of the occlusal sur-
face in locations where they would be least expected; an unusual number,
or a peculiar distribution of the canals, etc.

Lack of Dentition. — Cases in which there is a total absence of
teeth have been reported. Guilford reports the case of a man fifty years
of age who never had teeth, the jaws not differing in appearance or form
from those of a person whose teeth have been extracted. The mother
of the subject had the usual number of teeth, but the grandmother and
an uncle were both edentulous and hairless from birth. J. Tomes
mentions two cases having been reported to him, and Linderer mentions
one.

On the other hand, instances of a third dentition have been reported,
but there is great possibility of such observation being erroneous. Teeth
belonging to the permanent set which may have remained unerupted
for years could readily be mistaken as predecessors of a third set, when
in after years they made their appearance.

Supernumerary Teeth. — All teeth appearing in the mouth in addition
to the normal number are designated as supernumerary teeth. These are
divided into two classes, those normal in size and form and those abnormal
in size and form. The first-named are most likely to be of the simple
class, incisors and cuspids, and they may occupy a regular position in
the arch or may be found inside the arch closely associated with teeth of
the same type. Supernumerary bicuspids and molars are sometimes

44° ANOMALIES.

present either in regular position in the arch or inside of it. When the
jaws are long enough to accommodate them, an additional molar may
appear back of the third molar, making four molars instead of three,
and cases in which two extra molars were thus placed have occasionally
been met with.

Supernumerary teeth of the second class or those which are abnormal
in size and form are usually inclined to be cone-shaped and small in size,
this in respect to the root as well as the crown. Supernumerary teeth
of this character are usually found in the incisor region, but it occasionally
happens that they are found in the molar district, but here, instead of
having a single cone for the crown, they are mostly made up of a number
of smaller cones resembling many small cusps on the occlusal surface.
The number of supernumerary teeth may vary from one to eight or ten.
In the latter instance they are usually scattered through the entire alveo-
lar border of the hard palate. As many as ten or twelve teeth thus
located have been reported.

Again, certain teeth are frequently missing from the arch. This
may be occasioned by delayed eruption, or it may be the result of im-
proper activity within the tooth-germ itself, so that the tooth may have
failed to develop. The teeth most frequently missing are the upper
lateral incisors. The probable reason for this lies in the fact that the
germs for these teeth are located very near the surface of the bone, and
in some instances they are not even protected by a thin layer of bone
over the follicle. Being thus situated, they are more or less exposed to
violence sufficient to destroy the germs and thus render development of
the teeth impossible. Lack of certain teeth in the mouth appears to be
to some extent an hereditary feature, the condition being transmitted
from parent to child. Anomalies as to the size of individual teeth are
frequently noticed. When this is the case, it does not usually include
the entire set, but is confined to one or two, usually to the same teeth on
each side. Teeth that are above the usual size are generally found in
persons of large build, but if all the teeth are proportionate in size they
cannot be included within the abnormal class. The upper incisors
are most frequently affected in this way. Cases have been reported in
which the central incisors in the upper jaw have been fully twice the size
which they should normally have been, and all the remaining teeth in
the mouth perfect as to shape and size. Accompanying the abnormal
condition it is usual to find the teeth thus affected more or less abnormal
in outline, but, notwithstanding this, retaining their form sufficiently
well to permit their proper classification.

SUPERNUMERARY TEETH.

441

There are likewise teeth that are deficient in size. This does not
refer to teeth all relatively small, as frequently found in persons of small
frame and stature. This anomalous condition is often present in the
upper lateral incisors and in the third molars. Teeth that are deficient
in size generally possess their normal shape.

While the crowns of the teeth are susceptible to the above variations,
the roots appear to be anomalous much more frequently and to a more
marked extent than are the crowns. Most important among these may
be mentioned flexions of the roots. Curvatures in the roots may be
found either in single-rooted teeth or in multi-rooted teeth, and the point
of flexion may be located either at the center of the root or near its apex,

Fig. 357. — Anomalous Roots.

or both of these points may be affected. Flexions in the roots of the
teeth present a great variety in form. A single curve in one direction
may be present or a number of curvatures in different directions. In
multi-rooted teeth the roots may be so flexed that they will entwine about
each other, and the overlapping portions may be or may not be fused.
Probably the most important cause for the flexions of the roots of the
teeth is delayed eruption, this being particularly true if some positive
force prevents the progress of the organ. The roots of the teeth are
frequently anomalous in regard to number. These may have the same
general form and the same approximate length as the normal root or
roots would be, but they are proportionately smaller in diameter. Cases
are on record in which the incisor teeth have had two distinct roots, but
probably the most frequent location for the multiplicity of roots is found
in connection with the third molar. While this tooth when normally
developed as to crown is usually supported by three roots, it sometimes
possesses five or six smaller roots.

44-'

ANOMALIES.

It not infrequently happens that the roots of the teeth are less in
number than they should normally be. This is usually brought about
by the blending of the roots, which occurs during the developmental
stage. This may be of two distinct kind c . In the first place, it may
be the result of the conversion of two or more pulp canals into one, or the
individual roots may be united by a body of cementum being interposed
between them. In the former instance a single pulp canal is mostly
found within the blended roots, while in the latter the number of canals
is usually normal. With this blending there is generally a line of demar-
cation between individual roots as they should normally exist, in the

Fie;. 352. — Fusion of Molars.

way of longitudinal depressions extending through the entire root from
cervical line to apex. This anomaly, like that of the multiplicity of roots,
is principally confined to the third molars, and is more frequent in the
upper jaw than in the lower.

Fusion and Concrescence. — The union of two or more teeth is
known as fusion or concrescence, and may occur either during the develop-
ment of the organ or after this process has been completed. When union
takes place during development, it is characterized as fusion; when it takes
place after the completion of this process, it is known as concrescence.
There seems to be but little doubt that fusion of the teeth occurs through
some irregularity in the tooth-germ or germs, the beginning of the develop-
mental process taking place generally between two germs and continuing
together until the complete calcification of the organs. Teeth thus
united may have an internal anatomy corresponding in nearly every
respect to two separate teeth, and, on the other hand, they may possess

FUSION AND CONCRESCENCE.

443

but a single pulp chamber and canal. Fusion may take place in the
roots of the teeth alone, when it is called partial fusion; but when it is
confined to the roots and crowns alike, it is classified as complete fusion.
The teeth most likely to be affected in this way are the upper incisors
and the second and third molars. Some distinction must be made be-
tween those teeth which are united by a layer of cementum, and those of

Fig. 353. — Fusion and Concrescence.

Fig. 354. — Fusion and Concrescence.

true fusion, the latter existing only when there has been a union between
dentin and/ dentin.

Concrescence. — In concrescence, the roots of the teeth only can be
affected, as the union takes place after the complete development of the
organ. The roots of one tooth become united to the roots of another
through an additional growth of cementum, this growth being sufficiently

444

ANOMALIES.

extensive to cause absorption of the alveolar septa by pressure. Follow-
ing this there is a resorption of the pericementum. The cemental tissues
of the teeth are then brought in contact, and coalescence gradually takes
place. Concrescence may take place not only between the roots of two
teeth, but between the various roots of an individual tooth, this resulting
in the same manner as above described by the destruction of the septa
within the tooth socket. When concrescence takes place, the process
is usually confined to the apices of the roots, although they may become
coalesced throughout their entire length. The teeth most commonly
affected by concrescence are the molars and bicuspids, from the fact
that the alveolar walls, particularly the septa about these teeth are espe-
cially thin, owing to the form and relative location of the roots.

Fig. 355. — Geminous Tooth.

Geminous Teeth (Fig. 355). — It occasionally happens that two
separate germs are confined within a single sacculus, and from this
results two teeth, either similar or -dissimilar in size and form. One of
the pair may be normal as to form and size, while the other may be much
below the normal size, but more or less perfect in outline. Geminous
or twin teeth may be united or entirely separate. This condition is
most frequently found in the molar teeth, although cases in which the
bicuspids and incisors have been thus affected are recorded. Teeth
thus formed must not be confounded with those in which fusion is the
anomaly. In geminous teeth a single sac contains two tooth-germs
from which result two similarly formed teeth; in fusion two follicles
coalesce, each of which contains its own germ.

Besides the foregoing, the roots of the teeth are subject to anomalies
in size, in some instances being abnormally small, in others abnormally
large. In the former they may nearly always be characterized as anoma-

GEMINOUS TEETH.

445

lous, but this is not always true of the latter. When the roots are abnor-
mally large, it is somewhat difficult to discriminate between an anomalous
and a pathologic condition, the latter usually being the case when the

t FiG. 356.

Fig. 357-

roots of individual teeth are affected. Roots, to be considered anoma-
lous in size, should in a measure retain their normal form, cases of hyper-
trophy (hypercementosis) usually resulting in the destruction of the
normal contours (Figs. 356 and 357).

INDEX.

Accessory palatal foramina, 18
Achromatin substance, 298
Achromatic spindle, 301
Adipose tissue, 349
Alveolar process, 41

development of, 45
Alveoli, 42
Alveolodental membrane, 73, 102, 403

blood supply to, 104

cells of, 404

fibers of, 406

histology of, 403

interfibrous elements of, 409

nerve supply to, 104
Amelification, 361
Ameloblasts, 243, 360, 362
Anatomy, macroscopic, 293

microscopic, 293
Angular artery, 13
Anomalies of the teeth, 437
Anterior palatal foramen, 18

nerve, 18

superior dental nerve, 97
Antrum of Highmore, 45
Apical foramina, description of, 198
Areolar tissue, 348
Artery, deep facial, 18, 32

inferior dental, 93

internal maxillary, 18, 115

lingual, 32

ranine, 32

sublingual, 32
Azygos uvulae, 23

Bicuspid, lower first, 180
second, 183
upper first, 133
second, 143
Blastoderm. 307

layers of, 307
Blood and lymph, 294, 326
development, 326
physiology, 327
plasm, 326
structure, 327
course of, from heart to cheeks, 15
from heart to hard palate, 18
from heart to lips, 8
from heart to soft palate, 23
from heart to tongue, t,^
supply to the teeth, 91
Bone, 315

canaliculi of, 316
cells of, 316

Bone, Haversian canals of, 316

histologic examination of, 316

hyoid, 25, 59

inferior maxillary, 51

lacunae of, 316

marrow of, 318

matrix of, 315

osteoblasts of, 318

palate, 47

periosteum of, 318

Sharpey's fibers of, 317

superior maxillary, 34
Bones, of the mouth, 34

superior maxillary, 34
Branchial arches, 411
Brown stria; of Retzius, 355
Buccal cavity, 413

embryology of, 412

glands, 105, 341

orifice, 2
Buccinator muscle, 9

Calcification, beginning of, 268

of cementum, 282, 395, 439

of dentin, 268, 377

on enamel, 361
Canal, inferior dental, 55

infra-orbital, 35

posterior palatal, 38
Canaliculi, 316
Canals, Haversian, 316

pulp, 198
Canine eminence, 35

fossa, 35
Capsular ligament, 64
Cartilage, 313

cells of, 314

elastic, 314

fibro-, 314

hyaline, 314

matrix of, 313

Meckel's, 57, 261, 421

permanent, 315

temporary, 315

varieties of, 313
Cell, amitosis, 303

conductivity, 300

definition, 296

division, 302

fat, 313

g'and, 313

growth, 299

irritability, 299

karyokinesis, ^02

447

44 8

INDEX.

Cell, membrane, 295, 298

metabolism, 297, 299

mitosis, 302

motion, 300

nucleus, 294, 297, 298

origin, 296

plasma, 313

pigment, 313

protoplasm, 294, 295, 297

spontaneous generation, 296

wall, 298

vital manifestations, 299 ■
Cement corpuscles, 391

fibers, 393
Cementification, 282, 395
Cementoblasts, 282, 405
Cementum, 72, 385

calcification of, 282, 395

canaliculi of, 386

fibers of, 393

histology of, 385

lacunas of, 386

lamella?, 390

matrix of, 386
Central incisor, lower, 173

upper, no
Centrosome, 297
Cervical line, 77
Cheeks, 8

blood supply to, 13

external covering of, 9

glands of, 9, 105, 341

integument of, 9

internal covering of, 9

mucous membrane of, 9, 330

muscles of, 9

muscular tissue of, 348

nerves of, 15

substance of, 9
Chromosomes, 302
Chromatin substance, 298
Circumvallate papilla;, 27
Close skein, 302
Ccelom, 311

Concrescence of teeth, 487
Condyle, neck of, 56
Condyloid process, 56-63

forms of, 63
Connective tissue, cells of, 312

classification of, 312

fibrous, 312

intercellular substance of, 312

origin, 310
Coronoid process, 56
Crowns, anomalous, 485
Cuspid, lower, 177

upper, 124
Cytoplasm, 297

Decalcification, 290

Deciduous lower central incisor,
cuspid, 235
first molar, 231, 236
lateral incisor, 228, 234

234

Deciduous lateral incisor, measurement, 227

second molar, 233, 238

teeth, decalcification of, 227, 290
detail description of, 227
enamel organs for, 243
general description of, 224
occlusion of, 226
pulp chambers and canals of, 238

upper central incisor, 227
cuspid, 229
Deep facial artery, 18, 23

branches of, 23
Dendrites, 322
Dental arch, 77

arrangement of teeth in, 78
curve described by, 82
influence of temperament on, 83

follicle, 242, 258

formula, 75
Dental furrow, 264

pulp, 396

sacculus, 258
Dentin, 72, 368

calcification of, 377

cells, 257, 258

chemical analyses of, 369

exposed by dissection, 266

fibers of, 368, 372

granular layer, 376

history of, 368

matrix of, 369

organ, 242, 256

papillae, 257, 377

tubules of, 368, 370
Dentinal fibers, 372

sheaths, 368, 371

tubules, 368, 370
walls of, 371
Dentinification, 377
Dentition, completion of, 290

lack of, 439
Depressor anguli oris, 12

labii inferioris, 7

labii superioris, 6
Development of enamel, 358

of permanent teeth, 276

of teeth, 240, 470
Deutoplasm, 298
Diaster stage, 302
Digastric fossa, 53
Duct, parotid, 108

sublingual, 109

submaxillary, 108

Ectoderm, 307

Elastic cartilage, 314

Embryology, general, 293, 300, 304

of mouth and teeth, 411
Enamel, 72, 350

ameloblasts of, 360

brown stria; of, 355

calcification of, 361

cells, 243, 361

chemic composition of, 351

INDEX.

449

Enamel, cuticle of, 401
development of, 358
histology of, 350
organ, 242, 243, 398
cells of, 243, 250
external epithelium of, 243, 359
form of, 243, 245
internal epithelium of, 243, 359
stellate reticulum of, 243, 359
stratum intermedium of, 243, 359
prisms of, 351, 362
rods, formation of, 351
Endomysium, 320
Endothelium, 326
Endoplasm, 298
Entoderm, 307
Epimysium, 320
Epithelial cells, 309

tissues, 294, 309
Eruption of the teeth, 282
External maxillary artery, 13
oblique line, 52
pterygoid, 70
Exoplasm, 298

Facial angle, 88

artery, 15, 32
Facial artery, branches of, 13

nerve, 15

branches of, 15
Falciform papilla?, 27
Fat- cells, 313
Fauces, anterior pillars of, 20

isthmus of, 20

pillars of, 20

posterior pillars of, 20
Fibers, 297

dentinal, 372

Sharpey's 407
Fibroblasts, 406
Fibro-cartilage, 314
Fibrous connective tissue, 312
Fifth nerve, 96

division of, 96
Filiform papillae, 27
Floor of the mouth, boundaries of, 25

embryology of, 414

framework of, 25
Follicle, dental, 242, 258
Foramen, anterior palatal, 18

apical, description of, 198

cascum, 27

incisive, 41

infra-orbital, 35, 36

mental, 53
Foramina, accessory palatal, 18

posterior palatal, 18
Fossa, glenoid, 62
Frenas of the mouth, 100
Fungiform papillae, 27
Fusion of teeth, 442

and concrescence, 442

complete, 443

partial, 443
29

Ganglion, Gasserian, 95
Meckel's 19
sphcnopalatal, 19
Geminous teeth, 444
Genial tubercles, 53
Geniohyoglossus, 29
Germinal cells, 304

layers, 307
Germs for permanent molars, 258
Gills, 411
Gingivae, 17
Gingival border, 17

margins, outlines of, 100
Gland cells, 313
follicles, 339
parotid, 106, 344
saccular, 339
tubular, 339
sublingual, 109, 345
submaxillary, 108, 344
Glands and ducts of the mouth, histology of,
339
buccal, 105, 341

excretory ducts of, 340
labial, 105, 338
lingual, 106, 339
molar, 105, 340
of the cheeks, 341
of the hard palate, 341
of the mouth, 105, 339
of the soft palate, 106, 341
palatal, 106, 341
salivary, 106, 343
Glenoid fossa, 62
Gomphosis, 73
Groove, infra-orbital, 36
Ground substance, 294
Gubernaculum, 272

foramina of, 278
Gums, 99

epithelium of, 333
fibrous tissue of, ^^3
general description of, 99
mucous membrane of, 100, 332

Hard palate, 16

arch of, 17

blood supply to, 18

bones of, 17

covering of, 18

formation of, 18

glands of, 17, 341

mucous membrane of, 17, 334

nerves of, 19
Hare-lip, 435
Haversian canals, 316
Heredity, 302
Highmore, antrum of, 45
Histology, 293
Histogenesis, 293, 307, 304
Hooke, Robert, 294
Hyaline cartilage, 350
Hygolossus, 28
Hyoid bone, 59

45°

INDEX.

Hyoid development, 60

greater cornua, 59

lesser cornua, 59

muscles attached to, 60
Hypoblast, 457
Hyaloplasm, 298

Incisive foramen, 41

fossa, 36, 52
Incisor crest, 41

lower central, 173

lateral, 176
upper central, no
lateral, 118
Inferior coronary artery, 8, 13
vein, 13
dental artery, 93

incisive branch of, 95
mental branch of, 95
canal, 56
meatus, 40
nerve, 98
lingualis, 32
maxillary bone, 51
body of, 52
development of, 57
facial surface of, 5 2
internal surface of, 53
muscles attached to, 57
vertical portion of, 55
palatal vein, 18
turbinated crest, 37, 40
Infra-orbital canal, 36
foramen, 36
groove, 36
Intercellular substance, 311
Interglobular spaces, 376
Internal maxillary artery, 18, 91

infra-orbital branch of the, 91
superior maxillary branch of the, 91
oblique line, 53
pterygoid, 70
[nterproximate spaces, 83
Involuntary muscular tissue, 319
Isthmus of fauces, 20

Labial glands, 105, 340
Lacrimal canal, 38

tubercle, 38
Lacuna?, 316
Lateral incisor, upper, 113

lower, 176
Layer, granular, 376

Malpighi's, 430
Levator akeque nasi. 4

anguli oris, n

labii inferioris, 6
superioris, 6

palati, 22
Lines of Schreger, 357
Lingual artery, 32

glands, 105, 379

vein, 33
Lips, 2

blood supply to, 7

Lips, external covering of, 3
frenae of, 3, 100
glands of, 3, 105, 340
integument of, 3
internal covering of, 3
mucous membrane of, 3, 329
muscles of, 3
muscular tissue of, 347
nerves of, 8
substance of, 3
Loose skein, 302
Lower bicuspid, first, 180

buccal surface of, 181
calcification of, 180
cusps of, 180
distal surface of, 182
lingual surface of, 182
measurements of, 180
mesial surface of, 182
neck of, 182
occlusal surface of, 180
pulp cavity of, 221
root of, 183
second, 183

buccal surface of, 184
calcification of, 183
distal surface of, 186
lingual surface of, 185
measurements of, 183
mesial surface of, 185
neck of, 186

occlusal surface of, 183
root of, 186
bicuspids, general description of the, 180

pulp cavities of, 221
cuspid, 177

calcification of, 177
cusp of, 179
distal surface of, 178
labial surface of, 177
lingual surface of, 178
measurements of, 177
mesial surface of, 178
neck of, 179
root of, 179
cuspids, pulp cavities of, 220
incisor, central, 173

calcification of, 173
cervical margin of, 175
cutting-edge of, 175
rlistal surface of, 17^
labial surface of, 173
lingual surface of, 173
mesial surface of, t 74
measurements, 173
neck of, 176
root of, 176
incisors, pulp cavities of, 219
lateral incisor, general description of, 176
molar, first, 186

buccal surface of, 189
calcification of, 186
cusps of, 188
distal surface of, 191
lingual surface of, 190

INDEX.

451

Lower molar, first, measurements of, 186
mesial surface of, 190
neck of, 191
occlusal surface of, 186
roots of, 191
second, 192

buccal surface of, 193
calcification of, 192
distal surface of, 195
lingual surface of, 194
measurements of, 192
mesial surface of, 194
occlusal surface of, 192
roots of, 195
third, calcification of, 195

general description of the, 195
roots of, 196
types of the, 197
molars, pulp cavities of, 222

Malpighi's layer, 430
Mandible, 51

evolution of, 261
Masseter, n, 89
Mastication, muscles of, 68

active organs of, 1
Matrix, 311
Maxillary bones, development of, 260

sinus, 45
superior, 34
Meckel's cartilage, 59, 261, 421, 466

ganglion, 19
Median raphe, 27, 102
Membrana eboris, 257, 398

praformativa, 362

propria, 313
Mental foramen, 53

protuberance, 52
Mesoderm, 307

parietal, 311

visceral, 311
Meraplasm, 298
Middle meatus, 40

superior dental nerve, 97
Microsomes, 298
Moral glands, 341

lower, first, 186
second, 192
third, 195

upper, first, 146
second, 158
third, 165
Monaster stage, 302
Morula, 305
Motion, 300

amoeboid, 300

ciliary, 300

contractile, 300
Mouth, 1

angles of, 2

bones of, 34

boundaries of, 1

contents of, 1

development, 411

dissection of, 15

Mouth, divisions of, 15

entrance to, 1

epithelium of, 328

floor of, 16, 24

frenoe of, 100

general description of, 1

glands of, 102, 339

inferior portion of, 15

interior of, 15

lateral walls of, 8

mucous membrane of, 101, 328

muscular tissues of, 347

posterior boundary of, 20

primitive, 411

roof of, 15, 16

situation of, 1

submucosa of, 329

superior portion of, 15, 16

tunica propria of, 329

vestibule of, 1

walls of, 8
Mucous membrane of the checks, histology

of, 33 x
of gums, histology of, 332
of lips, histology of, 329
of mouth, 101, 328
blood supply to, 329
histology of, 329
nerve supply to, 329
of tongue, histology of, 335
Muscle, external pterygoid, 70
internal pterygoid, 70
masseter, n, 68
temporal, 69
Muscles, angular series, 20
of mastication, 68
of soft palate, 20
of tongue, 28
Muscular tissue, 318

tissues of cheek, 348

endomvsium of, 356
involuntary, 319
of lips, 347

perimysium of, 320
sarcolemma of, 320
sarcoplasm of, 320
of mouth, histology of, 347

non-striated, 319
of soft palate, 348

striated, 320
of tongue, 349
voluntary, 320
Mylohyoid ridge, 53
Myrtiform fossa, 36

Nasal crest, 41

spine, 41
Nasmyth's membrane, 385, 401
Nerve, anterior palatal, 19

cell, 321

corpuscles, 324

fiber, axis cylinder of, 324
neurilemma of, 324

fibers, 324

medullated, 325

452

INDEX.

Nerve fibers, non-medullated, 325

inferior dental, 98
maxillary, 98

middle superior dental, 97

posterior superior dental, 97

process, 323

superior dental, 97

superior maxillary, 96
Nerves, 324

endoneurium of, 325

epineurium of, 325

funiculi of, 324

medullary sheath of, 325

perineurium of, 322

of tongue, 33

system of, 322
Nervous tissue, 321
Neumann's sheath, 371
Neurilemma, 325
Neuroblasts, 322
Neurocyte, 321
Neuron, 321

Non-striated muscular tissues, 319
Nucleus, 294, 297, 298
Nuclear membrane, 298, 307

reticulum, 298

stain, 296
Nucleoplasm, 297
Nucleolus, 299

Occlusion of the teeth, 85

of the deciduous teeth, 226
Odontoblastic cells, 257
Odontoblasts, 257, 379, 398

processes of, 499
Oral cavity, 411

embryology of, 411

plate, 411

sinus, 41 1
Orbicularis oris, 4
Organogenesis, 293
Organs and tissues, 303
Osteoblasts, 354, 404
Osteoclasts, 406
Overbite, 88
Ovum, 296, 304

fertilization, 304

maturation, 305

segmentation, 305

Palatal glands, 106, 341
raphe, 16, 102
ruga?, 16, 102
Palate, bone, articulation of, 50

attachment of muscles to, 50
blood supply to, 50
development of, 50
horizontal plate of, 48
vertical plate of, 48
bones, 47
hard, 15, 16
soft, 19
Palatoglossus, 20
Palatomaxillary suture, 18
Palatopharyngeus, 22

238

Papilkc of the tongue, 27
Paraplasm, 298
Parotid duct, 108

gland, 106, 343, 382
Perichondrium, 350
Perimysium, 320
Periosteum, 318

Permanent incisors, papillae for, 278
molars, germs for, 258
teeth, 73

preparation for development of, 276
advance of, 290
Phagocytes, 300
Pigment cells, 313
Pillars of fauces, 20
Plasma cells, 313
Posterior foramina, 18
palatal canal, 38
superior dental nerve, 96
Primitive dental furrow, 264
Process, alveolar, 41
condyloid, 56
coronoid, 56
fronto-nasal, 411
Protoplasm, 294, 297
Protoplasmic stains, 296
Pulp, blood-vessels of, 400
canals, 198

cavities, description of, 198
dissections to show, 198
horns of, 199
of deciduous teeth,
of lower teeth, 219
of teeth, 198
of upper teeth, 200
cells of, 379
histology of, 379
nerves of, 400
odontoblasts of, 397

Quadratus menti, 7

Ranine artery, 32
Raphe, 16, 27, 102
Retzius, brown slriaj of, 355
Ridge, mylohyoid, 53
Risorius muscle, 12
Roots, anomalous, 485
formation of, 282
Ruga, 16, 102

Sacculus, dental, 258, 307
Salivary glands, 106, 343

blood-vessels of, 345

nerves of, 345
Sarcolemma, 320
Sarcoplasm, 320
Schleiden, 294
Schreger, lines of, 395
Schultze, Max, 296
Schwann, 295

sheath of, 362

white substance of, 362
Seventh nerve, 15
Sharpey's fibers, 407

INDEX.

453

Sheath, medullary, 322

of Schwann, 323
Simple saccular glands, 339

tubular glands, 339
Soft palate, 19

blood supply to, 23
glands of, 341
muscles of, 20
muscular tissue of, 334, 348
nerves of, 23
substance of, 20
Spaces, interglobular, 376

interproximate, 83
Spermatozoon, 304
Sphenomaxillary ligament, 64
Sphenopalatal ganglion, 19
Spongioplasm, 298
Stellate reticulum, 243, 359
Stenson, foramen of, 41
Stratum intermedium, 243 359
Striated muscular tissue 356
Styloglossus, 31
Stylomaxillary ligament, 65
Sublingual artery, 32
duct, 109
fossa, 53
gland, 108
Submaxillary duct, 108
fossa, 53
gland, 108
Submucosa, 329
Supernumerary teeth, 439
Superior coronary artery, 8, 13
vein, 13
lingualis, 32

maxillary bone, articulation of, 44
alveolar process of, 41
blood supply to, 44
development of, 44
facial surface of, 35
malar process of, 40
muscles attached to, 44
nasal process of, 39
orbital surface of, 34
palatal process of, 37, 40
proximal surface of, 37
sinus of, 45
tuberosity of, 39
zygomatic surface of, 39
bones, 34
nerve, 96
meatus, 40
palatal vein, 18
turbinated crest, 40
Suture, palatomaxillary, 18
Symphysis, 52

Teeth, 72

anterior, 76

apical extremities of, 76
articulation of, 85
attachment of, 72
blood supply to, 91-95
classification of, 72, 73
complex, 72

Teeth, deciduous, 73, 224

description in detail of, no

development of, 240, 428, 470

dissections of, 198

division of, 7 2

eruption of, 334

names of, 75

nerve supply to, 95

occlusion of, 85

permanent, 73

posterior, 76

pulp cavities of, 198

roots of, 75

simple, 72

surfaces of, 75

tissues of, 350

veins from, 95
Temporal muscle, 69
Temporomandibular articulation, 61

movements of, 66
Tensor palati, 23
Tissues of the body, divisions of, 294

of the teeth, 350
Tissue, adipose, 313

areolar, 312

connective, 294, 310

elementary, 294, 303

epithelial, 294, 309

muscular, 294, 318

nervous, 294, 321
Tongue, 25

attachment of, 26

base of, 26

blood-vessels of, 32

circumvallate papillas of, 336

dorsum of, 26

filiform papilte of, 335

frenum of, 27

function of, 26

fungiform papilte of, 336

glands of, 106, 342

median raphe of, 27

mucous membrane of, 335

muscles of, 28

muscular tissue of, 349

nerves of, ^^

papillas of, 27

post-tip, 26

prebase of, 26

shape of, 26

size of, 26

substance of, 26
Tonsil, 20
Tonsillar recess, 20
Tooth band, 241

development, cellular stage of, 240, 428
saccular stage of, 260, 266

follicle, walls of, 259

fundamental parts of a, 72
general description of a, 72

germs, 242

roots, preparations for development of,
282

sac, 258

sacs exposed by dissection, 264

454

INDEX.

Tooth sockets, 42

tissues of a, 72
Transverse facial artery, 13

branches of, 13
vein, 13
Tunica propria, 365

Upper bicuspid, first, 133

occlusal surface, 134
angles of, 138
buccal surface of, 136
calcification of, 133
crown of, 134
cusps of, 135
distal surface of, 138
measurements of, 133
mesial surface of, 137
neck of, 138
lingual surface of, 137
pulp cavity of, 206
roots of, 139
types of, 140

second, calcification of, 143
general description of, 144
measurements of, 143
pulp cavities of, 209
cuspid, 124

calcification of, 124

crown, 125

cusp of, 1 29

cutting-edge of, 129

deciduous, 229

distal surface of, 128

labial surface of, 125

measurement of, 124

mesial surface of, 127

neck of, 130

lingual surface of, 126

pulp cavity of, 204

root of, 130

types of, 130
incisor, central, no

calcification of, 1 10
cervical margin, 115
crown of, in
cutting-edge of, 114
developmental grooves, no
distal surface of, 114
labial surface of, 111
measurements of, no
mesial surface of, 113
neck of, 115
lingual surface of, 112
occlusal surface, 115
pulp cavity of, 200
root of, 116
types of, 116

lateral, 118
angles, 122
calcification of, 118
crown of, 119
cutting-edge of, 121
deciduous, 228
distal surface of, 121
labial surface of, 119

Upper incisor, lateral, lingual surface of, 120

measurements of, 118

mesial surface of, 121
neck of, 122

pulp cavity of, 203

root of, 122

types of, 123
molar, first, 146

buccal surface of, 152

calcification of, 146

cusps of, 149

deciduous, 231

distal surface of, 154

fossa? and grooves of, 151

lingual surface of, 153

marginal ridges of, 147

measurements of, 146

mesial surface of, 154

neck of, 155

occlusal surface of, 147

pulp cavity of, 213

roots of, 155

types of, 156
second, 158

deciduous, 233

angles of, 164

buccal surface of, 162

calcification of, 158

cusps of, 160
deciduous, 233

distal surface of, 163

fossa? and grooves of, 162

lingual surface of, 162

marginal ridges of, 159

measurement of, 158

mesial surface of, 163
neck of, 164

occlusal surface of, 159

pulp cavity of, 216

roots of, 164
third, 165

buccal surface of, 167

calcification of, 165

cusps of, 169

distal surface of, 166

fossa? and grooves of, 170

lingual surface of, 167

marginal ridges, 169

measurements of, 165

mesial surface of, 166

occlusal surface of, 168

pulp cavity of, 216

types of, 171
Uvula, 20

Vein, inferior palatal, 18

lingual, 33

superior palatal, 18
Visceral arches, 411

furrows, 411
Voluntary muscular tissue, 320

Zygomaticus major, 12
minor, 7